Surface-treated steel sheet

ABSTRACT

A surface-treated steel sheet includes a steel sheet; and a plating layer which is formed on one surface or both surfaces of the steel sheet and which includes zinc and one of the group consisting of vanadium and zirconium, wherein the plating layer includes dendrite-shaped crystals including metallic zinc, and intercrystal filling regions which fill spaces between the dendrite-shaped crystals and show amorphous diffraction patterns when electron beam diffraction is carried out, wherein when the plating layer includes the vanadium, the intercrystal filling regions include a hydrated vanadium oxide or a vanadium hydroxide, and, wherein when the plating layer includes zirconium, the intercrystal filling regions include a hydrated zirconium oxide or a zirconium hydroxide.

TECHNICAL FIELD OF THE INVENTION

The present invention relates to a surface-treated steel sheet having excellent corrosion resistance (barrier property) of plating in corrosive environments and excellent coating film adhesion.

The present application claims Japanese Patent Application No. 2015-116554 and Japanese Patent Application No. 2015-116604 filed on Jun. 9, 2015, the contents of which are incorporated herein by reference.

RELATED ART

From the related art, electrogalvanized steel sheets have been used in a variety of fields such as home appliances, construction materials, and automobiles. For electrogalvanized steel sheets, there is a demand for further improving the corrosion resistance (hereinafter, barrier property) of plating in corrosive environments.

As a method for improving the barrier property of electrogalvanized steel sheets, an increase in the plating amount (basis weight) of galvanized layers is considered. However, when the basis weight of a galvanized layer is increased, there has been a problem in that the manufacturing costs increase and the workability or weldability degrades.

In addition, as a method for improving the barrier property or external appearance of electrogalvanized steel sheets, thus far, techniques of forming coating films on surfaces have been widely used. However, when the adhesion (coating film adhesion) between plating layers and coating films in electrogalvanized steel sheets is insufficient, in spite of the formation of coating films on surfaces, the effect of the formation of coating films cannot be sufficiently obtained. Therefore, there is a demand for not only improving the barrier property of electrogalvanized steel sheet but also improving the coating film adhesion.

In recent years, studies have been underway regarding the improvement of the barrier property by adding a vanadium element to galvanized layers in electrogalvanized steel sheets. For example, Non Patent Documents 1 to 4 describe techniques of electrocrystallizing Zn—V composite oxides on the surface of a copper sheet which is a negative electrode.

Patent Document 1 describes a technique of forming a V-thickened layer in a surface layer portion of a plating layer in a zinc-based plated steel sheet.

Patent Document 2 describes a technique regarding a plating layer including zinc and vanadium and having a plurality of dendrite-shaped arms.

Patent Document 3 describes that a plating layer formed on a steel sheet and including zinc and vanadium has dendrite-shaped crystals in which a vanadium oxide is present in zinc and describes that, in portions other than the dendrite-shaped crystals, phases having a higher vanadium content ratio than the dendrite-shaped crystals are present.

Patent Document 4 describes that, in a zinc-based composite electroplated steel sheet including zinc and a vanadium hydroxide, a vanadium hydroxide is coprecipitated in zinc.

PRIOR ART DOCUMENT Patent Document

[Patent Document 1] Japanese Unexamined Patent Application, First Publication No. 2013-185199

[Patent Document 2] Japanese Patent No. 5273316

[Patent Document 3] Japanese Unexamined Patent Application, First Publication No. 2013-108183

[Patent Document 4] Japanese Unexamined Patent Application, First Publication No. 2011-111633

Non Patent Document

[Non-Patent Document 1] CAMP-ISIJ Vol. 22 (2009) 933 to 936

[Non-Patent Document 2] Iron and steel Vol. 93 (2007) No. 11, pp. 49 to 54

[Non-Patent Document 3] The Surface Finishing Society of Japan, The summary of the 115th Lectures, 9A-26, pp. 139 and 140

[Non-Patent Document 4] Bulletin of The Iron and Steel Institute of Japan, Vol. 13, No. 4, p. 245, 2008. 4. 1

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention

However, for surface-treated steel sheets of the related art having a plating layer including zinc and vanadium on the surface of a steel sheet, there has been a demand for further improving the barrier property.

In addition, since vanadium (V) is a rare element, there is a desire for platings having an excellent barrier property which replace zinc-vanadium platings.

The present invention has been made in consideration of the above-described circumstances, and an object of the present invention is to provide a surface-treated steel sheet having an excellent barrier property and excellent coating film adhesion in which a plating layer including zinc and vanadium or zirconium is formed on the surface of a steel sheet.

Means for Solving the Problem

In order to solve the above-described problems, the present inventors repeated intensive studies as described below.

That is, the present inventors formed plating layers including zinc and vanadium or zirconium on the surfaces of steel sheets under a variety of conditions using an electroplating method and the steel sheets as a negative electrode and investigated the barrier property and the coating film adhesion.

As a result, the present inventors found that it is preferable to form a plating layer which includes zinc and vanadium and has dendrite-shaped crystals including metallic zinc and intercrystal filling regions including a hydrated vanadium oxide or a vanadium hydroxide. The above-described plating layer has intercrystal filling regions including a hydrated vanadium oxide or a vanadium hydroxide and thus has a higher corrosion potential and a superior barrier property compared with, for example, plated steel sheets formed by providing a galvanized layer instead of this plating layer.

In addition, the present inventors found that, under certain conditions, phases including a hydrated zirconia oxide or a zirconia hydroxide are formed in the peripheries of the dendrite-shaped crystals made of metallic zinc. It was clarified that the plating layer has an equal or better barrier property and excellent coating film adhesion compared with zinc-vanadium platings, which led to the completion of the present invention. Individual aspects of the present invention are as described below.

(1) A surface-treated steel sheet according to an aspect of the present invention includes a steel sheet; and a plating layer which is formed on one surface or both surfaces of the steel sheet and which includes zinc and one of the group consisting of vanadium and zirconium; in which the plating layer includes dendrite-shaped crystals including metallic zinc, and intercrystal filling regions which fill spaces between the dendrite-shaped crystals and show amorphous diffraction patterns when electron beam diffraction is carried out, in which when the plating layer includes the vanadium, the intercrystal filling regions include a hydrated vanadium oxide or a vanadium hydroxide, and, in which when the plating layer includes zirconium, the intercrystal filling regions include a hydrated zirconium oxide or a zirconium hydroxide.

(2) In the surface-treated steel sheet according to (1), a constitution in which, when the plating layer includes the vanadium, V/Zn which is a molar ratio of the vanadium to the zinc in the intercrystal filling regions is 0.10 or more and 2.00 or less, and, in which when the plating layer includes the zirconium, Zr/Zn which is a molar ratio of the zirconium to the zinc in the intercrystal filling regions is 1.00 or more and 3.00 or less may be employed.

(3) In the surface-treated steel sheet according to (1) or (2), a constitution in which the plating layer includes the vanadium and surface layers of the dendrite-shaped crystals include a zinc oxide or a zinc hydroxide may be employed.

(4) The surface-treated steel sheet according to any one of (1) to (3) further includes a base-material plating layer having Zn/V, which is a molar ratio of the zinc to the vanadium, of 8.00 or more between the steel sheet and the plating layer.

(5) The surface-treated steel sheet according to any one of (I) to (4), further incluses an organic resin film having a polyurethane resin and 1% to 20% by mass of carbon black on a surface of the plating layer.

(6) A method for manufacturing the surface-treated steel sheet according to the aspect of the present invention is a method for manufacturing the surface-treated steel sheet according to any one of (1) to (5), the method including: a base-material-forming process of forming protrusions and recesses by precipitating a hydrated vanadium oxide or a vanadium hydroxide on the steel sheet by carrying out an electroplating at a current density of 0 to 18 A/dm² using a plating bath containing 0.10 to 4.00 mol/l of Zn²⁺ ions and 0.01 to 2.00 mol/l of V ions or 0.10 to 4.00 mol/l of Zr ions; and an upper layer plating process of carrying out an electroplating on the steel sheet on which the protrusions and the recesses are formed at a current density of 21 to 200 A/dm² using the plating bath.

Effects of the Invention

According to the respective aspects, it is possible to provide a surface-treated steel sheet having an excellent barrier property and excellent coating film adhesion.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross sectional view showing an example of a surface-treated steel sheet according to a first embodiment.

FIG. 2 is a schematic cross sectional view showing an example of a surface-treated steel sheet according to a second embodiment.

FIG. 3 is a schematic view showing an example of a plating apparatus that is used to manufacture the surface-treated steel sheet according to the present embodiment.

FIG. 4A is a schematic view showing the precipitation of a vanadium compound in a process of manufacturing the surface-treated steel sheet shown in FIG. 1.

FIG. 4B is a schematic view showing the growth of dendrite-shaped crystals in the process of manufacturing the surface-treated steel sheet shown in FIG. 1.

FIG. 4C is a schematic view showing the generation of hydrogen at front ends of branch portions of the dendrite-shaped crystals in the process of manufacturing the surface-treated steel sheet shown in FIG. 1.

FIG. 5A is a cross sectional photograph of a plating layer in a surface-treated steel sheet of Example V4 captured in the entire thickness direction using a transmission electron microscope (TEM).

FIG. 5B is an enlarged photograph of an interface portion between a steel sheet and a plating layer in the cross section of FIG. 5A.

FIG. 5C is an enlarged photograph of dendrite-shaped crystals and peripheral portions thereof in the cross section of FIG. 5A.

FIG. 6 is a scanning electron microscopic (SEM) photograph of the plating layer in the surface-treated steel sheet of Example V4.

FIG. 7 is a scanning electron microscopic (SEM) photograph of a plating layer in a surface-treated steel sheet of Comparative Example x2.

FIG. 8 is a photograph showing electron beam diffraction images of the plating layer in the surface-treated steel sheet of Example V4.

FIG. 9 is a transmission electron microscopic (TEM) photograph of a plating layer in a surface-treated steel sheet of Example Z4.

EMBODIMENTS OF THE INVENTION First Embodiment, Surface-Treated Steel Sheet 10”

Hereinafter, a surface-treated steel sheet 10 of a first embodiment when a plating layer contains vanadium will be described in detail with reference to the accompanying drawings.

FIG. 1 is a schematic cross sectional view showing an example of the surface-treated steel sheet 10 according to the present embodiment. In the surface-treated steel sheet 10 shown in FIG. 1, a base-material layer 20, a plating layer 30, and a surface layer 40 are sequentially formed on each of both surfaces of the steel sheet 1 from the steel sheet 1 side. FIG. 1 shows only the base-material layer 20, the plating layer 30, and the surface layer 40 formed on one surface (upper surface) side of the steel sheet 1 and does not show those on the other surface (lower surface) side.

In the present embodiment, the steel sheet 1 having the plating layer 30 formed on the surface is not particularly limited. For example, as the steel sheet 1, any types of steel sheets such as an extremely low C-type (a ferrite-dominant structure) steel sheet, an Al-k-type (a structure including pearlite in ferrite) steel sheet, a two-phase structure-type (for example, a structure including martensite in ferrite or a structure including bainite in ferrite) steel sheet, a working-induced transformation-type (a structure including residual austenite in ferrite) steel sheet, or a fine crystal-type (a ferrite-dominant structure) steel sheet may be used.

The base-material layer 20 may be provided between the steel sheet 1 and the plating layer 30 as shown in FIG. 1. The base-material layer 20 is provided as necessary in order to improve the adhesion between the steel sheet 1 and the plating layer 30. In the present embodiment, the base-material layer 20 which has a thickness of 1 to 300 nm and is made of crystals including nickel is preferably provided.

As shown in FIG. 1, the plating layer 30 has dendrite-shaped crystals 31 and intercrystal filling regions 32 which are disposed between the dendrite-shaped crystals 31 and shows amorphous diffraction patterns when electron beam diffraction is carried out.

In the present invention, “being amorphous” means that, when electron beam diffraction is carried out on each layer in the cross sectional direction using a transmission electron microscope (TEM), diffraction patterns attributed to crystal structures are not obtained.

The intercrystal filling region 32 includes a hydrated vanadium oxide or a vanadium hydroxide. The intercrystal filling region 32 preferably includes a vanadium hydroxide since the coating film adhesion is improved.

In addition, the intercrystal filling region 32 preferably includes zinc. When the intercrystal filling region 32 includes zinc, the corrosion resistance improves.

When the intercrystal filling region 32 includes a hydrated vanadium oxide or a vanadium hydroxide and zinc, the molar ratio (V/Zn) of vanadium to zinc in the intercrystal filling region 32 is preferably 0.10 or more and 2.00 or less. When the molar ratio (V/Zn) is in the above-described range, and the intercrystal filling regions show amorphous diffraction patterns when electron beam diffraction is carried out, excellent corrosion resistance (barrier property) and coating film adhesion can be obtained. When the molar ratio (V/Zn) of vanadium to zinc in the intercrystal filling region 32 is less than 0.10, there are cases in which amorphous diffraction patterns cannot be stably obtained, and the corrosion resistance deteriorates. On the other hand, when the molar ratio exceeds 2.00, the sacrificial protection property of platings deteriorates.

As shown in FIG. 1, a plurality of the dendrite-shaped crystals 31 is formed in the plating layer 30. The shapes of the plurality of dendrite-shaped crystals 31 may be different from one another, or some of them may have the same shape. The shape of each of the dendrite-shaped crystals 31 may be a needle shape or a rod shape. In addition, the respective dendrite-shaped crystals 31 may be crystals which extend linearly in the length direction or extend in a curved manner. The cross sectional shapes of the respective dendrite-shaped crystals 31 are not particularly limited, and examples thereof include a circular shape, an elliptical shape, a polyhedral shape, and the like. In addition, the cross sectional shapes of the respective dendrite-shaped crystals 31 may be even or uneven in the length direction. In addition, the outer circumference dimensions of the respective dendrite-shaped crystals 31 may be even or uneven in the length direction.

In the surface-treated steel sheet 10 of the present embodiment, the respective dendrite-shaped crystals 31 have an inside 3 a of the dendrite-shaped crystal and a surface layer 3 b formed on the surface of the dendrite-shaped crystal 31 as shown in FIG. 1. The inside 3 a of the dendrite-shaped crystal 31 grows from the steel sheet 1 side toward the outside and has a plurality of branch portions. The surface layer 3 b is formed in a substantially uniform thickness so as to cover the surface of the inside 3 a of the dendrite-shaped crystal 31.

The dendrite-shaped crystal 31 having the inside 3 a of the dendrite-shaped crystal 31 and the surface layer 3 b which is shown in FIG. 1 preferably has a maximum length of 4.0 μm or less and a maximum cross sectional width of 0.5 μm or less. When the maximum length and the maximum width of the dendrite-shaped crystal 31 are in the above-described ranges, the plating layer 30 has fine dendrite-shaped crystals 31 and becomes dense. Therefore, the barrier action of the plating layer 30 improves and a superior barrier property can be obtained. In order to further improve the barrier property, the maximum length of the dendrite-shaped crystal 31 is more preferably 3.0 μm or less. In addition, the maximum cross sectional width of the dendrite-shaped crystal 31 is more preferably 0.4 μm or less.

In the present embodiment, “the maximum length of the dendrite-shaped crystal 31” is obtained by observing a cross section of the plating layer using a scanning electron microscope (SEM), measuring the maximum lengths of 50 dendrite-shaped crystals 31, and computing the average value thereof.

In addition, “the maximum cross sectional width of the dendrite-shaped crystal 31” is obtained by observing a cross section of the plating layer using a transmission electron microscope (TEM), measuring the maximum widths of 50 dendrite-shaped crystals 31, and computing the average value thereof.

The inside 3 a of the dendrite-shaped crystal 31 preferably includes metallic zinc. In addition to the metallic zinc, the inside 3 a of the dendrite-shaped crystal 31 may include other metal components having a higher precipitation potential than zinc such as nickel.

In addition, the surface layer 3 b preferably includes crystals including a zinc oxide or a zinc hydroxide. The surface layer 3 b more preferably includes the crystals of a zinc oxide. The thickness of the surface layer 3 b is preferably 0.1 to 500 nm.

In addition, as shown in FIG. 1, the inside 3 a of the dendrite-shaped crystal 31 may include granular crystals 3 c. The granular crystal 3 c includes zinc and nickel. The grain diameters of the granular crystals 3 c are preferably 0.1 to 500 nm. When the grain diameters of the granular crystals 3 c are in the above-described range, superior coating film adhesion can be obtained.

In the surface-treated steel sheet 10 of the present embodiment, Zn/V which is the molar ratio of zinc to vanadium included in the plating layer 30 is preferably 0.50 or more and less than 8.00. When Zn/V is 0.50 or more and less than 8.00, vanadium being included provides a superior barrier function, which is preferable.

In the surface-treated steel sheet 10, a base-material plating layer (not shown) containing zinc may be formed between the steel sheet 1 and the plating layer 30 (between the base-material layer 20 and the plating layer 30 when the base-material layer 20 is formed). This is because the base-material plating layer (not shown) being formed provides an excellent corrosion resistance improvement effect which is attributed to the sacrificial protection by zinc.

The base-material plating layer (not shown) may include zinc and vanadium and may have Zn/V, which is the molar ratio of the zinc to the vanadium, of 8.00 or more. In addition, the base-material plating layer (not shown) may also be constituted of zinc alone.

As an upper layer of the plating layer 30, an upper layer plating layer (not shown) containing zinc may be formed. The upper layer plating layer (not shown) being formed provides an excellent corrosion resistance improvement effect which is attributed to the sacrificial protection by zinc, which is preferable.

The upper layer plating layer (not shown) may also be constituted of zinc alone. In addition, the upper layer plating layer (not shown) includes zinc and vanadium and may have Zn/V, which is the molar ratio of the zinc to the vanadium, of 8.00 or more.

The base-material plating layer (not shown) can be formed between the steel sheet 1 and the plating layer 30 (between the base-material layer 20 and the plating layer 30 when the base-material layer 20 is formed) by controlling the current density for electroplating and the adjusting the molar ratio of zinc to vanadium.

The upper layer plating layer (not shown) can be formed on the plating layer 30 using the same method for the base-material plating layer (not shown).

The molar ratio (a/b) of the amount of zinc (a) included in the insides 3 a of the dendrite-shaped crystals 31 to the total of the amounts of zinc (b) included in the intercrystal filling regions 32 and the surface layers 3 b of the dendrite-shaped crystals 31 is preferably in a range of 0.10 or more and 3.00 or less.

When the molar ratio (a/b) is 0.10 or more, the sacrificial protection action of the metallic zinc in the dendrite-shaped crystals 31 can be effectively obtained when flaws are generated on the surface of the plating layer 30, and a superior barrier property can be obtained. In order to more effectively obtain the sacrificial protection action of the metallic zinc in the dendrite-shaped crystals 31, the molar ratio (a/b) is more preferably set to 0.20 or more.

In addition, when the molar ratio (a/b) is 3.00 or less, the barrier property improvement action of the steel sheet 1 which is attributed to the zinc oxide or the zinc hydroxide in the surface layers of the dendrite-shaped crystals 31 which does not easily transmit the air or water can be effectively obtained, and a superior barrier property can be obtained. In order to more effectively obtain the barrier property improvement action of the surface layers 3 b of the dendrite-shaped crystals 31, the molar ratio (a/b) is more preferably 0.25 or less.

In addition, the molar ratio (A/B) of the total (A) of the amount of zinc included in the dendrite-shaped crystals 31 and the amount of zinc included in the surface layers 3 b of the dendrite-shaped crystals 31 to the amount of vanadium (B) included in the intercrystal filling regions 32 is preferably 0.05 or more and 6.00 or less. When the molar ratio (A/B) is 0.05 or more, the sacrificial protection action of the metallic zinc in the dendrite-shaped crystals 31 and the barrier property improvement action of the zinc oxide or the zinc hydroxide in the surface layers 3 b of the dendrite-shaped crystals 31 are effectively obtained, and a superior barrier property can be obtained.

In order to more effectively obtain the sacrificial protection action of the dendrite-shaped crystals 31 and the barrier property improvement action of the surface layers 3 b of the dendrite-shaped crystals 31, the molar ratio (A/B) is more preferably 0.10 or more. In addition, when the molar ratio (A/B) is 6.00 or less, an effect of improving the barrier property by the corrosion potential set to a positive potential by containing vanadium is more effectively showed. In order to further improve the barrier property improvement action of vanadium being included, the molar ratio (A/B) is more preferably 5.00 or less and more preferably 4.50 or less.

In the present embodiment, the amount of vanadium in the plating layer 30 is preferably 1% by mass to 20% by mass. When the amount of vanadium in the plating layer 30 is 1% by mass or more, a superior barrier property can be obtained. The amount of vanadium in the plating layer 30 is more preferably 4% by mass or more in order to further improve the barrier property and the coating film adhesion. When the amount of vanadium in the plating layer 30 is 20% by mass or less, the amount in the dendrite-shaped crystals 31 and the surface layers 3 b of the dendrite-shaped crystals 31 become relatively great, and the sacrificial protection action of the dendrite-shaped crystals 31 and the barrier property improvement action of the surface layers 3 b of the dendrite-shaped crystals 31 can be effectively obtained.

The amount of vanadium in the plating layer 30 is more preferably 15% by mass or less in order to ensure the amount in the dendrite-shaped crystals 31 and the surface layers 3 b of the dendrite-shaped crystals 31.

The adhered amount of the plating layer 30 is preferably 1 g/m² or more and more preferably 3 g/m² or more in order to improve the barrier property. In addition, the adhered amount of the plating layer 30 is preferably 90 g/m² or less, more preferably 50 g/m² or less, and still more preferably 15 g/m² or less. When the adhered amount of the plating layer 30 is 15 g/m² or less, the amount of metal being precipitated is smaller compared with that in electrogalvanizing of the related art (generally, approximately 20 g/m²), and the adhered amount is economically excellent from the viewpoint of metal costs or electric power costs for forming the plating layer 30.

In the surface-treated steel sheet 10 of the present embodiment, the natural immersion potential (corrosion potential) when the plating layer 30 is immersed in a 5% aqueous NaCl solution at 25° C. as a working electrode is preferably −0.8 V or more. The corrosion potential is preferably 0.2 V or more higher (positive) than those of plated steel sheets (the corrosion potentials are approximately −1.0 V) formed by providing a galvanized layer instead of the plating layer 30. In order to further improve the barrier property, the corrosion potential is more preferably −0.7 V or more.

As shown in FIG. 1, the surface layer 40 made of one or more films is formed on the surface of the plating layer 30. The surface layer 40 is provided as necessary. The surface layer 40 being formed improves the corrosion resistance.

One or more films forming the surface layer 40 preferably contain an organic resin (R).

The organic resin (R) in the film is not particularly limited, and examples thereof include polyurethane resins.

As the organic resin (R) in the film, one or more organic resins (not modified) may be mixed and used, or one or more organic resins obtained by modifying at least one organic resin in the presence of at least one different organic resin may be mixed and used.

Examples of the polyurethane resin that is used as the organic resin (R) include resins obtained by reacting a polyol compound and a polyisocyanate compound and then elongating chains using a chain elongation agent.

The polyol compound that is used as a raw material of the polyurethane resin is not particularly limited as long as the polyol compound has two or more hydroxyl groups per molecule, and examples thereof include ethylene glycol, propylene glycol, diethylene glycol, 1,6-hexanediol, neopentyl glycol, triethylene glycol, glycerin, trimethylolethane, trimethylolpropane, polycarbonate polyol, polyester polyol, polyether polyols such as bisphenol hydroxypropyl ether, polyesteramide polyol, acryl polyol, polyurethane polyol, and mixtures thereof.

As the polyisocyanate compound that is used as a raw material of the polyurethane resin, compounds having two or more isocyanate groups per molecule are used, and examples thereof include aliphatic isocyanates such as hexamethylene diisocyanate (HDI), alicyclic diisocyanates such as isophorone isocyanate (IPDI), aromatic diisocyanates such as torylene diisocyanate (TDI), aromatic aliphatic diisocyanates such as diphenylmethane diisocyanate (MDI), and mixtures thereof

As the chain elongation agent that is used to manufacture the polyurethane resin, compounds having one or more active hydrogen atoms per in the molecule can be used, and examples thereof include aliphatic polyamines such as ethylenediamine, propylenediamine, hexamethylenediamine, diethylenetriamine, dipropylenetriamine, triethylenetetramine, and tetraethylenepentamine, aromatic polyamines such as torylenediamine, xylylenedimaine, and diaminodiphenylmethane, alicyclic polyamines such as diaminocyclohxylmethane, pyperazine, 2,5-dimethylpyperazine, and isophorodiamine, hydrozines such as hydrazine, succinic dihydrazide, adipic dihydrazide, and phthalic dihydrazide, alkanoleamines such as hydroxyethyl diethylenetriamine, 2-[(2-aminoethyl)amino]ethanol and 3-aminopropanediol, and the like. These compounds that are used as the chain elongation agent can be used singly, or two or more compounds can be used as a mixture.

In addition, the polyurethane resin that is used as the organic resin (R) may be a polyurethane resin obtained by heating a raw material solution including a blocked isocyanate compound and the polyol compound to a temperature at which a block agent is dissociated and reacting regenerated isocyanate groups and the polyol component of the polyol compound in the raw material solution.

The blocked isocyanate compound is a compound that regenerates isocyanate groups when heated to a temperature or higher at which the block agent is dissociated. As the blocked isocyanate compound, it is possible to use, for example, compounds obtained by masking the isocyanate groups in the polyisocyante compound with a well-known block agent of the related art. As the block agent, it is possible to use, for example, dimethylpyrazole (DMP), methyl ethyl ketone oxime, and the like.

One or more films forming the surface layer 40 preferably include, in addition to the organic resin (R), one or more raw materials selected from a phosphoric acid compound (P), an organic silicon compound (W), carbon black (C), a fluoro metal complex compound (F), and polyethylene wax (Q).

The phosphoric acid compound (P) in the film is more preferably a compound that emits phosphate ions in the film. When the phosphoric acid compound (P) is a compound (P) that emits phosphate ions in the film, when a coating composition for forming the films during the formation of the films is brought into contact with the plating layer 30 or phosphate ions derived from the phosphoric acid compound are eluted from the films after the formation of the films, the phosphoric acid compound (P) and a vanadium oxide present on the surface of the plating layer 30 react with each other, and a poorly-soluble phosphoric acid-vanadium-based film is formed on the surface of the plating layer 30. Therefore, the white rust resistance can be significantly improved.

When the phosphoric acid compound (P) is a insoluble compound that does not emit phosphate ions in the environment, the phosphoric acid compound (P) in the film inhibits the migration of corrosion factors such as water and oxygen, and thus an excellent barrier property can be obtained.

As the phosphoric acid compound (P) in the film, it is possible to use, for example, phosphoric acids such as orthophosphoric acid, metaphosphoric acid, pyrophosphoric acid, triphosphoric acid, and tetraphosphoric acid or ammonium dihydrogen phosphate. These phosphoric acid compounds (P) may be used singly, or two or more phosphoric acid compounds may be jointly used.

The amount of the phosphoric acid compound (P) in the film is preferably 1% to 20% by mass, more preferably 6% to 18% by mass, and most preferably 10% to 15% by mass in terms of phosphate ions. When the concentration of phosphate ions in the film is 1% by mass or more, an excellent barrier property can be obtained. In addition, when the concentration of phosphate ions in the film is 20% by mass or less, the swelling of coating film caused by the elution of phosphoric acid can be prevented.

When the phosphoric acid compound (P) is included in the film, a barrier layer (not shown) including vanadium in the plating layer 30 and the phosphoric acid compound in the films and having an excellent barrier property against corrosion factors (water, oxygen, and the like) is formed on the surface of the steel sheet 1. As a result, compared with a case in which the surface layer 40 is not formed on the surface of the plating layer 30, the white rust resistance is excellent, and an effect of delaying the generation of red rust can be obtained, and thus the barrier property significantly improves.

Examples of the organic silicon compound (W) in the film include hydrolysis condensates of silane coupling agents and the like.

Examples of silane coupling agents that are used to generate the organic silicon compound (W) in the film include 3-glycidoxypropyl trimethoxysilane, 3-aminopropyl triethoxysilane. The silane coupling agents may be used singly or two or more silane coupling agents may be jointly used.

The organic silicon compound (W) in the film is preferably a compound obtained from a reaction between a silane coupling agent (W1) containing an amino group and a silane coupling agent (W2) containing an epoxy group. In this case, dense films having a high crosslinking density are formed from a reaction between the amino group and the epoxy group and a reaction between alkoxysilyl groups that are respectively included in the silane coupling agent (W1) and the silane coupling agent (W2) or partial hydrolysis products thereof. As a result, the barrier property, flaw resistance, and contamination resistance of the surface-treated steel sheet further improve.

Examples of the silane coupling agent (W1) containing an amino group include 3-aminopropyltriethoxysilane. Examples of the silane coupling agent (W2) containing an epoxy group include 3-glycidoxypropyltrimethoxysilane.

The molar ratio {(W1)/(W2)} of the silane coupling agent (W1) containing an amino group to the silane coupling agent (W2) containing an epoxy group is preferably 0.5 or more and 2.5 or less and more preferably 0.7 or more and 1.6 or less. When the molar ratio {(W1)/(W2)} is 0.5 or more, a sufficient film production property can be obtained, and thus the barrier property improves. In addition, when the molar ratio is 2.5 or less, a sufficient water resistance can be obtained, and thus an excellent barrier property can be obtained.

The number-average molecular weight of the organic silicon compound (W) in the film is, for example, preferably 1,000 or more and 10,000 or less and more preferably 2,000 or more and 10,000 or less. When the number-average molecular weight of the organic silicon compound (W) is 1,000 or more, the film becomes excellent in terms of the water resistance, and the alkali resistance and the barrier property become more favorable. On the other hand, when the number-average molecular weight of the organic silicon compound (W) is 10,000 or less, it is possible to stably dissolve or disperse the organic silicon compound (W) in aqueous media which contain water as a main component, and there are cases in which the storage stability degrades.

As the method for measuring the number-average molecular weight of the organic silicon compound (W), direct measurement by means of time of flight mass spectrometry (TOF-MS) may be used, or conversion measurement by means of chromatography may be used.

The mass ratio (R/W) of the organic resin (R) to the organic silicon compound (W) in the film is preferably 1.0 to 3.0. When R/W is 1.0 or more, cohesion failure does not easily occur in the film during working, and the working adhesion becomes favorable. In addition, when R/W is 3.0 or less, the effect of the inclusion of the organic silicon compound (W) can be sufficiently obtained, and films having high hardness can be obtained.

The organic silicon compound (W) can be manufactured using, for example, a method in which the above-described silane coupling agent is dissolved or dispersed in water and is stirred at a predetermined temperature for a predetermined period of time, thereby obtaining hydrolysis condensates.

The film containing the organic silicon compound (W) can be formed by, for example, manufacturing an aqueous liquid or alcohol-based liquid containing the organic silicon compound (W) as a raw material of the coating composition for forming the film and applying and drying the coating composition including the liquid on the plating layer.

The aqueous liquid or alcohol-based liquid containing the organic silicon compound (W) can be manufactured using, for example, a method in which the organic silicon compound such as the hydrolysis condensate of the silane coupling agent is dissolved or dispersed in water, thereby obtaining aqueous liquids, a method in which the organic silicon compound such as the hydrolysis condensate of the silane coupling agent is dissolved in an alcohol-based organic solvent such as methanol, ethanol, or isopropanol, thereby obtaining alcohol-based liquids, or the like.

When the aqueous liquid or alcohol-based liquid containing the organic silicon compound (W) is manufactured, in addition to the organic silicon compound (W) and water or the alcohol-based organic solvent, acids, alkalis, organic solvents, surfactants, and the like may be added thereto in order to dissolve or disperse the silane coupling agent or the hydrolysis condensate thereof in the aqueous liquid or alcohol-based liquid. Particularly, the pH of the aqueous liquid or alcohol-based liquid is preferably adjusted to 3 to 6 by adding organic acids in addition to water or the alcohol-based organic solvent from the viewpoint of the storage stability of the aqueous liquid or alcohol-based liquid.

The solid content concentration of the organic silicon compound (W) in the aqueous liquid or alcohol-based liquid of the organic silicon compound (W) is preferably 25% by mass or less. When the solid content concentration of the organic silicon compound (W) is 25% by mass or less, the storage stability of the aqueous liquid or alcohol-based liquid becomes favorable.

In the film, the carbon black (C) is preferably included as a coloring pigment. When the carbon black is included in the film, fine unevenness present on the surface of the plating layer is covered, a beautiful black external appearance is obtained, and excellent designability can be obtained.

Examples of the carbon black (C) in the film include well-known carbon blacks such as furnace black, ketjenblack, acetylene black, and channel black. In addition, as the carbon black (C) in the film, carbon black on which a well-known ozone treatment, plasma treatment, or liquid-phase oxidation treatment is carried out may be used.

The particle diameters of the carbon black (C) in the film are not particularly limited as long as there are no problems with the dispersibility in the coating composition for forming the film, the qualities of coating films, and the coatability. When dispersed in water-based solvents, the carbon black coheres together in the process of dispersing the carbon black. Therefore, generally, it is difficult to disperse the carbon black in water-based solvents while maintaining the primary particle diameters. Therefore, the carbon black in the coating composition for forming the film is present in a form of secondary particles having larger particle diameters than the primary particle diameters. Therefore, the carbon black in the film formed using the coating composition is, similar to those in the coating composition, present in a form of secondary particles.

As the carbon black that is used as the raw material of the film, for example, carbon black having primary particle diameters of 10 nm to 120 nm can be used. When the designability and barrier property of the film are taken into account, the particle diameters of the carbon black in the film are preferably 10 nm to 50 nm.

In order to secure the designability and barrier property of the film, the particle diameters of the carbon black in a form of secondary particles which is dispersed in the film are important. The average particle diameter of the carbon black in the film is preferably 20 nm to 300 nm.

The amount of the carbon black (C) in the film is, for example, preferably 1% to 20% by mass, more preferably 3% to 15% by mass, and most preferably set to 5% to 13% by mass. When the amount of the carbon black (C) in the film is 1% by mass or more, an even black external appearance can be obtained. In addition, when the amount of the carbon black (C) in the film is 20% by mass or less, it is possible to ensure the amount of raw materials other than the carbon black (C) in the film, and thus an excellent barrier property can be obtained.

In the film, the fluoro metal complex compound (F) may be included. The fluoro metal complex compound (F) acts as a crosslinking agent in the film and improves the cohesive force of the film. The fluoro metal complex compound (F) is not particularly limited, and a fluoro metal complex compound having titanium is preferably used from the viewpoint of the barrier property. Examples of the fluoro metal complex compound (F) include hexafluorotitanic acid.

In the film, the polyethylene wax (Q) may be included. The polyethylene wax (Q) is capable of improving the flaw resistance of the film. Therefore, when the polyethylene wax (Q) is included in the film, the lubricity of the surface-treated steel sheet enhances, the friction resistance attributed to the contact between, for example, the steel sheet and a press die decreases, and it is possible to prevent damages in worked portions of the steel sheet and scratches being generated during the handling of the steel sheet.

The polyethylene wax (Q) in the film is not particularly limited, and well-known lubricants can be used. Specifically, as the polyethylene wax (Q), polyolefin resin-based lubricants are preferably used.

The polyolefin resin-based lubricants that are used as the polyethylene wax (Q) are not particularly limited, and examples thereof include hydrocarbon-based wax such as polyethylene.

The amount of the polyethylene wax (Q) in the film is preferably 0.5% by mass or more and 10% by mass or less and more preferably 1% by mass or more and 5% by mass or less in the film. When the amount of the polyethylene wax (Q) is 0.5% by mass or more, a flaw resistance improvement effect can be obtained. When the amount of the polyethylene wax (Q) is 10% by mass or less, it is possible to ensure the amount of raw materials other than the polyethylene wax (Q) in the film, and thus an excellent barrier property can be obtained.

“Method for Manufacturing Surface-Treated Steel Sheet 10”

Next, a method for manufacturing the surface-treated steel sheet 10 will be described.

A method for manufacturing the surface-treated steel sheet of the present embodiment includes a base-material-forming process of forming protrusions and recesses by precipitating a hydrated vanadium oxide or a vanadium hydroxide on the steel sheet 1 by carrying out an electroplating at a current density of 0 to 18 A/dm² using a plating bath containing 0.10 to 4.00 mol/l of Zr²⁺ ions and 0.01 to 2.00 mol/l of V ions or 0.10 to 4.00 mol/l of Zr ions and an upper layer plating process of carrying out an electroplating on the steel sheet 1 on which the protrusions and the recesses are formed at a current density of 21 to 200 A/dm² using the plating bath. The above-described base-material-forming process is a factor that affects V/Zn which is the molar ratio of the vanadium to the zinc in the above-described intercrystal filling regions in the plating layer. When the current density in the base-material-forming process exceeds 18 A/dm², V/Zn which is the molar ratio of vanadium to zinc in the intercrystal filling regions reaches less than 0.10.

In the present embodiment, a pretreatment is carried out on both surfaces of the steel sheet 1 forming the plating layer 30 as necessary. As the pretreatment, it is preferable to provide 1 to 300 nm-thick nickel platings on both surfaces of the steel sheet 1 and form the base-material layers 20.

Next, the plating layer 30 is formed on one surface or both surfaces of the steel sheet 1. The present embodiment will be described using a method in which the plating layers 30 are formed on both surfaces of the steel sheet 1 by means of electroplating using a plating apparatus shown in FIG. 3 as an example.

FIG. 3 is a schematic view showing an example of the plating apparatus. In the present embodiment, out of rolls 4 a, 4 b, 5 a, and 5 b, the rolls 4 a and 4 b disposed above the steel sheet 1 function as connection portions (conductors) that electrically connect a power supply (not shown) and the steel sheet 1. The steel sheet 1 is electrically connected to the rolls 4 a and 4 b and acts as a negative electrode. When electroplating is carried out, a plurality of the plating apparatuses shown in FIG. 3 is arranged in series and used. The base-material-forming process is carried out in the plating apparatus shown in FIG. 3 or in a region surrounded by the rolls 4 a and 5 a and the intermediate branching paths 2 d and 2 f in FIG. 3. In addition, the upper layer plating process is carried out in the plating apparatus shown in FIG. 3 or in a region surrounded by the intermediate branching paths 2 d and 2 f and the rolls 4 b and 5 b in FIG. 3.

A plating tank 21 has an upper portion tank 21 a disposed above the steel sheet 1 and a lower portion tank 21 b disposed below the steel sheet 1.

As shown in FIG. 3, at locations adjacent to the steel sheet 1 in the upper portion tank 21 a and the lower portion tank 21 b, a plurality of positive electrodes 3 made of platinum or the like is disposed at predetermined intervals from the steel sheet 1. The respective positive electrodes 3 are disposed so that the surfaces of the positive electrodes facing the steel sheet 1 become substantially parallel to the surface of the steel sheet 1. The respective positive electrodes 3 are electrically connected to the power supply (not shown) using non-shown connection members.

The upper portion tank 21 a and the lower portion tank 21 b are filled with a plating bath 2. As shown in FIG. 3, the steel sheet 1 migrating with the surface direction set to be substantially horizontal is disposed between the upper portion tank 21 a and the lower portion tank 21 b of the plating tank 21. In addition, the steel sheet 1 being passed through the plating tank 21 in an arrow direction using the rolls 4 a, 4 b, 5 a, and 5 b is in a state of being immersed in the plating bath 2 in the upper portion tank 21 a and the lower portion tank 21 b. Therefore, in the present embodiment, the steel sheet 1 is transported using the rolls 4 a, 4 b, 5 a, and 5 b, and the steel sheet 1 is migrated in the plating bath 2, whereby the plating bath 2 falls into a fluid state in which the plating bath 2 is relatively fluid with respect to the steel sheet 1.

As shown in FIG. 3, in the upper portion tank 21 a, an upper portion supply pipe 2 a that supplies the plating bath 2 to the upper portion tank 21 a is provided so as to penetrate through the upper surface of the upper portion tank 21 a. The upper portion supply pipe 2 a is branched into a plurality of outer circumferential branching paths 2 c and the plurality of intermediate branching paths 2 d (only one path is shown in FIG. 3) in the upper portion tank 21 a. The plurality of intermediate branching paths 2 d is disposed along the width direction of the steel sheet 1 between the positive electrodes 3 adjacent to each other in a plan view. The intermediate branching path 2 d includes an opening portion that supplies the plating bath 2 toward between the positive electrodes 3 on both sides and the steel sheet 1. The plurality of outer circumferential branching paths 2 c is disposed along the width direction of the steel sheet 1 between the positive electrode 3 and the rolls 4 a and 4 b in a plan view. The outer circumferential branching path 2 c includes an opening portion that supplies the plating bath 2 toward between the positive electrode 3 and the steel sheet 1.

In the upper portion tank 21 a, a discharge opening (not shown) that discharges the plating bath 2 is provided and is connected to the upper portion supply pipe 2 a through a pipe including a pump (not shown). Therefore, in the upper portion tank 21 a, the plating bath 2 which has been supplied from the upper portion supply pipe 2 a and been discharged from the discharging opening turns into the plating bath 2 in a fluid state in which the plating bath is again supplied from the upper portion supply pipe 2 a through the pipe using the pump and is circulated.

In the lower portion tank 21 b, a lower portion supply pipe 2 b that supplies the plating bath 2 to the lower portion tank 21 b is provided so as to penetrate through the lower surface of the lower portion tank 21 b. The lower portion supply pipe 2 b is branched into a plurality of outer circumferential branching paths 2 e and the plurality of intermediate branching paths 2 f (only one path is shown in FIG. 3) in the lower portion tank 21 b. The plurality of intermediate branching paths 2 f is disposed along the width direction of the steel sheet 1 between the positive electrodes 3 adjacent to each other in a plan view. The intermediate branching path 2 f includes an opening portion that supplies the plating bath 2 toward between the positive electrodes 3 on both sides and the steel sheet 1. The plurality of outer circumferential branching paths 2 e is disposed along the width direction of the steel sheet 1 between the positive electrode 3 and the rolls 5 a and 5 b in a plan view. The outer circumferential branching path 2 e includes an opening portion that supplies the plating bath 2 toward between the positive electrode 3 and the steel sheet 1.

In the lower portion tank 21 b, a discharge opening (not shown) that discharges the plating bath 2 is provided and is connected to the lower portion supply pipe 2 b through a pipe including a pump (not shown). Therefore, in the lower portion tank 21 b, the plating bath 2 which has been supplied from the lower portion supply pipe 2 b and been discharged from the discharging opening turns into the plating bath 2 in a fluid state in which the plating bath is again supplied from the lower portion supply pipe 2 b through the pipe using the pump and is circulated.

When the electric conduction time in the base-material-forming process is adjusted to 0.05 seconds to 8.00 seconds, the intercrystal filling regions 32 stably show amorphous diffraction patterns.

In the present embodiment, it is assumed that the plating layer 30 is formed on the surface of the steel sheet 1 through a mechanism described below. FIG. 4A to FIG. 4C are schematic views showing the state of the surface of the steel sheet 1 in the process of manufacturing the surface-treated steel sheet 10 shown in FIG. 1.

In the plating apparatus shown in FIG. 3, the steel sheet 1 having a nickel plating layer (base-material layer) 20 a formed on the surface sequentially comes into contact with the plating bath 2 from a portion that has passed through between the rolls 4 a and 5 a, and plating is initiated at a current density of 18 A/dm² or less.

That is, the rolls 4 a and 5 a are rolls for electric conduction and are also referred to as conductor rolls. The steel sheet and a plating liquid come into contact with each other after passing through between these conductor rolls 4 a and 5 a.

In the present embodiment, on the surface (solid-liquid interface) of the steel sheet 1 on which the nickel plating layer 20 a is formed which has passed through the rolls 4 a and 5 a, before the precipitation of zinc, a vanadium compound 6 including a hydrated vanadium oxide or a vanadium hydroxide is precipitated as shown in FIG. 4A, and the base-material-forming process in which protrusions and recesses are formed is initiated.

This is assumed to be because, at the current density of 18 A/dm² or less, vanadium having a high precipitation potential is reduced and precipitated, but zinc having a low precipitation potential is not precipitated. Meanwhile, in the base-material-forming process, the vanadium compound 6 including a hydrated vanadium oxide or a vanadium hydroxide is precipitated. This base-material is different from the above-described base-material layer 20. This base-material is incorporated into the plating layer 30 in the end.

In the base-material-forming process, when the precipitation of the vanadium compound 6 on the surface of the steel sheet 1 is initiated, a plurality of current-concentrating portions 61 is formed on the surface of the steel sheet 1 as shown in FIG. 4A. The current-concentrating portions 61 can be assumed as portions which are made of portions in which the vanadium compound 6 is not or slightly precipitated on the surface of the steel sheet 1 and allow electric currents to easily flow.

When the current density is set to 21 A/dm² or more, the potential reaches the precipitation potential of Zn, and the reduction and precipitation reaction of zinc is initiated. The current-concentrating portions 61 act as the starting points, as shown in FIG. 4B, the dendrite-shaped crystals 3 a including metallic zinc grow, and the upper layer plating process is initiated. When the dendrite-shaped crystals 3 a grow, it is assumed that it becomes easier for the crystals to grow at the front end sections of the dendrite-shaped crystals 3 a.

In the upper portion plating process, electric currents further concentrate at the front ends of a plurality of branching portions branched from the dendrite-shaped crystals 3 a as the dendrite-shaped crystals 3 a grow, and, as shown in FIG. 4C, it is assumed that hydrogen 62 is generated at the solid-liquid interfaces between the front ends of the branching portions and the plating bath 2.

The hydrogen 62 generated in the above-described manner increases the pH of the solid-liquid interfaces between the surfaces of the dendrite-shaped crystals 3 a and the plating bath 2. As a result, it is assumed that crystals including a zinc oxide or a zinc hydroxide are precipitated so as to cover the surfaces of the dendrite-shaped crystals 31 and the dendrite-shaped crystals 31 having the surface layer 3 b shown in FIG. 1 are formed. In addition, it is assumed that, as the pH of the plating bath 2 increases, amorphous substances including a hydrated vanadium oxide or a vanadium hydroxide are precipitated between the dendrite-shaped crystals 31 adjacent to each other, and the intercrystal filling regions 32 shown in FIG. 1 are formed.

In the present embodiment, as described above, in the base-material-forming process, the electric conduction time is controlled to a range of 0.05 to 8.00 seconds. Therefore, before the precipitation of zinc on the surface of the steel sheet 1, the precipitation of the vanadium compound 6 is initiated, and the plurality of current-concentrating portions 61 is formed on the surface of the steel sheet 1. As a result, it is assumed that, through the above-described mechanism, the dendrite-shaped crystals 31 are obtained, and the intercrystal filling regions 32 showing amorphous diffraction patterns when electron beam diffraction is carried out are obtained. The migration time of the steel sheet 1 passing through the interval D is more preferably in a range of 1.00 to 6.00 seconds.

When the electric conduction time in the base-material-forming process is shorter than 0.05 seconds, the precipitation amount of the vanadium compound 6 precipitated before the precipitation of zinc on the surface of the steel sheet 1 is insufficient. Therefore, it becomes difficult for the dendrite-shaped crystals 31 made of metallic zinc to grow in the current-concentrating portions 61 formed on the surface of the steel sheet 1. In addition, the intercrystal filling regions 32 including a hydrated vanadium oxide or a vanadium hydroxide are not obtained or the amorphous diffraction patterns become unstable even when the intercrystal filling regions 32 are obtained.

When the electric conduction time in the base-material-forming process exceeds 8.00 seconds, the precipitation amount of the vanadium compound 6 precipitated before the precipitation of zinc on the surface of the steel sheet 1 becomes too great, and thus the number of the current-concentrating portions 61 formed on the surface of the steel sheet 1 becomes smaller or zero. Therefore, it becomes difficult for the dendrite-shaped crystals 31 made of metallic zinc to grow, and the dendrite-shaped crystals 31 and the intercrystal filling regions 32 are not obtained or the amorphous diffraction patterns become unstable even when the intercrystal filling regions 32 are obtained.

In the present embodiment, in the base-material-forming process, electroplating is preferably carried out under a condition in which the current density reaches 0 to 18 A/dm², and electroplating is more preferably carried out under a condition in which the current density reaches 2 to 15 A/dm². When the current density is set to 18 A/dm² or less in the base-material-forming process, the molar ratio (V/Zn) of vanadium to zinc in the intercrystal filling regions 32 reaches 0.10 or more and 2.00 or less, the intercrystal filling regions 32 show amorphous diffraction patterns when electron beam diffraction is carried out, and consequently, the barrier property and the coating film adhesion can be improved. On the other hand, when the current density is not in the above-described range in the base-material-forming process, the intercrystal filling regions 32 are not formed or the amorphous diffraction patterns become unstable even when the intercrystal filling regions 32 are obtained.

In addition, in the upper layer plating process, electroplating is preferably carried out under conditions in which the current density reaches 21 to 200 A/dm². When the current density is set to 21 A/dm² or more, it is possible to sufficiently generate the hydrogen 62 in the solid-liquid interfaces between the front ends of the branching portions of the dendrite-shaped crystals 31 and the plating bath 2. Therefore, the precipitation amount of the hydrated vanadium oxide or the vanadium hydroxide in the intercrystal filling regions 32 increases. Therefore, it is possible to form the plating layer 30 in which the amount of vanadium is great and the barrier properties are excellent. In addition, when the current density exceeds 200 A/dm², plating structures become coarse or cracks are likely to be generated, and thus there is a concern that the adhesion between the plating layer 30 and the steel sheet 1 may degrade.

The average flow rate of the plating bath 2 in the plating tank 21 during plating is preferably in a range of 20 to 300 m/min and more preferably in a range of 40 to 200 m/min. When the average flow rate of the plating bath 2 is in a range of 20 to 300 m/min, it is possible to prevent the generation of cracks in the plating layer 30 and supply ions to the surface of the steel sheet 1 from the plating bath 2 without any hindrance.

As the plating bath 2, a plating bath including a V compound and a Zn compound is used. Meanwhile, to the plating bath 2, in addition to the V compound and the Zn compound, a pH adjuster, metal compounds other than the V compound and the Zn compound, and additives may be added as necessary.

Examples of the pH adjuster include H₂SO₄, NaOH, and the like.

Examples of the additives include Na₂SO₄ and the like which stabilize the electric conductivity of the plating bath 2.

Examples of other metal compounds include nickel compounds such as NiSO₄.6H₂O and the like. When the plating bath 2 includes a nickel compound, the concentration of Ni²⁺ in the plating bath 2 is preferably 0.01 mol/l or more. In such a case, the plating layer 30 including a sufficient amount of nickel can be formed. The plating layer 30 including nickel is capable of providing excellent plating adhesion, which is preferable.

Examples of the Zn compound that is used in the plating bath 2 include metallic Zn, ZnSO₄.7H₂O, ZnCO₃, and the like. These Zn compounds may be used singly or two or more zinc compounds may be jointly used.

In addition, examples of the V compound that is used in the plating bath 2 include ammonium (V) metavanadate, potassium (V) metavanadate, sodium (V) metavanadate, VO(C₅H₇O₂)₂ (vanadyl acetylacetonate (IV)), VOSO₄.5H₂O (vanadyl sulfate (IV)), and the like. These V compounds may be used singly or two or more vanadium compounds may be jointly used.

As the plating bath 2, a plating bath including Zn²⁺ and VO²⁺ is preferably used.

When the plating bath 2 includes Zn²⁺, the concentration of Zn²⁺ is preferably 0.10 to 4.00 mol/l and more preferably 0.35 to 2.00 mol/l.

When the plating bath 2 includes VO²⁺, the concentration of VO²⁺ in the plating bath 2 is preferably 0.01 mol/l or more and less than 2.00 mol/l. When the plating bath 2 including VO²⁺ in the above-described range is used, it is possible to easily form the plating layer 30 in which the amount of vanadium is great and the barrier property is excellent. When the amount of VO²⁺ in the plating bath 2 is below the above-described range, it becomes difficult to ensure the amount of vanadium in the plating layer 30. In addition, when the amount of VO²⁺ in the plating bath 2 is above the above-described range, a large amount of expensive vanadium is used in the plating bath 2, which becomes economically disadvantageous.

In addition, as the plating bath 2, a plating bath including 0.10 mol/l or more of Na⁺ in the plating bath 2 is preferably used. In this case, it is possible to enhance the electric conductivity of the plating bath 2 and easily form the plating layer 30 in the present embodiment.

The temperature of the plating bath 2 is not particularly limited, but is preferably in a range of 40° C. to 60° C. in order to easily and efficiently form the plating layer 30 in the present embodiment.

In addition, the pH of the plating bath 2 is preferably in a range of 1 to 5 and more preferably in a range of 1.5 to 4 in order to easily form the plating layer 30 in the present embodiment.

In the present embodiment, after the formation of the plating layer 30, the surface layer 40 is preferably formed on the plating layer 30 as necessary by applying a treatment agent that improves a barrier property, fingerprint resistance, flaw resistance, lubricity, designability, and the like.

Through the above-described process, the surface-treated steel sheet 10 shown in FIG. 1 can be obtained.

“Second Embodiment, Surface-Treated Steel Sheet 210”

Hereinafter, a surface-treated steel sheet 210 of a second embodiment when a plating layer 230 contains zirconium will be described.

The surface-treated steel sheet 210 of the present embodiment includes a steel sheet 201 and a plating layer 230 formed on one surface or both surfaces of the steel sheet. The plating layer 230 includes zinc and zirconium. In addition, the plating layer 230 has dendrite-shaped crystals 231 including metallic zinc and intercrystal filling regions 232 including one or both of a hydrated zirconium oxide and a zirconium hydroxide. Hereinafter, the surface-treated steel sheet 210 will be described in detail.

The steel sheet 201 is the same as the steel sheet 1 in the first embodiment and thus will not be described.

As described above, the plating layer 230 has the dendrite-shaped crystals 231 including metallic zinc and the intercrystal filling regions 232 including one or both of a hydrated zirconium oxide and a zirconium hydroxide.

The dendrite-shaped crystal 231 is a dendrite-shaped crystal phase including metallic zinc, and the intercrystal filling region 232 includes one or both of a hydrated zirconium oxide and a zirconium hydroxide, is formed in the peripheries of the dendrite-shaped crystal 231, and has an amorphous pattern in electron beam diffraction.

The plating layer 230 has an aspect in which the dendrite-shaped crystals 231 are precipitated earlier, and then the intercrystal filling regions 232 are precipitated in the peripheries of the dendrite-shaped crystals 231.

As described above, the dendrite-shaped crystal 31 in the first embodiment has the inside 3 a and the surface layer 3 b. As described above, the inside 3 a of the dendrite-shaped crystal 31 preferably includes metallic zinc and may include other metal components such as nickel. On the other hand, the surface layer 3 b of the dendrite-shaped crystal 31 preferably includes a zinc oxide or a zinc hydroxide and more preferably include crystals of a hydrated zinc oxide. Meanwhile, the dendrite-shaped crystal 231 in the present embodiment does not have any insides and any surface layers.

The dendrite-shaped crystal 231 may be formed of metallic zinc alone and may include, in addition to metallic zinc, other metal components having a higher precipitation potential than zinc such as nickel. In addition, the dendrite-shaped crystals 231 grow from the steel sheet 201 side toward the plating layer 230 surface side along the thickness direction of the plating layer 230 and have a structure of being branched toward the surface of the plating layer 230. When the dendrite-shaped crystals 231 include metallic zinc, it is possible to impart a sacrificial protection property to the plating layer 230.

The intercrystal filling region 232 may include a zinc oxide in addition to one or both of the hydrated zirconium oxide and the zirconium hydroxide. When the intercrystal filling regions 232 include the above-described substances, it is possible to impart a barrier property to the plating layer 230. In addition, since the intercrystal filling regions 232 include the hydrated oxide or the hydroxide as a main body, it is possible to ensure coating film adhesion when coating films are formed in the intercrystal filling regions 232.

The intercrystal filling regions 232 show amorphous diffraction patterns when electron beam diffraction is carried out.

When the intercrystal filling region 232 includes the hydrated zirconium oxide or the zirconium hydroxide and a zinc oxide, the molar ratio (Zr/Zn) of zirconium to zinc in the intercrystal filling regions 232 is preferably 1.00 or more and 3.00 or less. When the molar ratio (Zr/Zn) is in the above-described range, and the intercrystal filling regions 232 show amorphous diffraction patterns when electron beam diffraction is carried out, an excellent corrosion resistance (barrier property) and excellent coating film adhesion can be obtained.

On the plating layer 230, an amorphous layer 250 showing amorphous diffraction patterns when electron beam diffraction is carried out may be formed.

The amorphous layer 250 is assumed to be a layer that is first formed during the formation of the plating layer 230. That is, it is assumed that the amorphous layer 250 is first formed on the steel sheet 201 and then the plating layer 230 including the dendrite-shaped crystals 231 and the intercrystal filling regions 232 grows between the steel sheet 201 and the amorphous layer 250.

The amorphous layer 250 is a layer including zirconium oxide as a main body and may include a small amount of zinc. The amorphous layer 250 shows a barrier property when formed on the plating layer 230.

After the formation of the plating layer 230, the steel sheet 201 having the plating layer 230 is immersed in an acidic solution, whereby the amorphous layer 250 can be removed. Through the above-described process, the amorphous layer 250 may be removed from the surface-treated steel sheet 201.

When the amorphous layer 250 is removed, the plating layer 230 is exposed. The surface of the plating layer 230 has a higher surface roughness than the amorphous layer 250 and superior coating film adhesion compared with a case in which the amorphous layer 250 is formed.

The adhered amount of the plating layer 230 is preferably 1 g/m² or more and preferably 3 g/m² or more in order to improve the barrier property. In addition, the adhered amount of the plating layer 230 is preferably 60 g/m² or less, more preferably 40 g/m² or less, and still more preferably 20 g/m² or less. When the adhered amount of the plating layer 230 is 20 g/m² or less, the amount of metal being precipitated is smaller compared with that in electrogalvanizing of the related art (generally, approximately 20 g/m²). In addition, when the adhered amount is too large, it becomes easy for cracks to be generated in the plating layer 230.

The thickness of the plating layer 230 is preferably in a range of 0.5 to 40 μm, more preferably in a range of 1.0 to 20 μm, and still more preferably in a range of 2.0 to 15 μm. When the thickness of the plating layer 230 is the lower limit or more, the barrier property can be improved. In addition, when the thickness of the plating layer 230 is the upper limit or less, it becomes difficult for cracks to be generated in the plating layer 230. The thickness of the plating layer 230 can be controlled by adjusting the amount of electric power that is conducted during electroplating.

The thickness of the amorphous layer 250 is preferably in a range of 0.20 to 2.00 μm, more preferably in a range of 0.30 to 1.50 μm, and still more preferably in a range of 0.50 to 1.00 μm. When the thickness of the amorphous layer 250 is the upper limit or more, it is possible to impart the barrier property to the plating layer 230. In addition, when the thickness of the amorphous layer 250 is the lower limit or less, it is possible to ensure the barrier property by preventing the generation of cracks. The thickness of the amorphous layer 250 can be controlled by adjusting the concentration of Zr in the plating bath during electroplating. That is, as the concentration of Zr in the plating bath during electroplating increases, it is possible to increase the thickness of the amorphous layer 250.

The plating layer 230 is made of, in terms of the average concentration, Zr: 3 to 40 atm %, Zn: 3 to 40 atm %, residual oxygen, and impurities. When the concentration of Zr in the plating layer 230 is 3 atm % or more, it is possible to enhance the barrier property. In addition, when the concentration of Zr in the plating layer 230 is 40 atm % or less, it is possible to ensure the barrier property by preventing the generation of cracks in the plating layer 230. In addition, when the concentration of Zn in the plating layer 230 is 3 atm % or more, it is possible to impart a sacrificial protection effect to the plating layer 230. In addition, when the concentration of Zn in the plating layer 230 is 40 atm % or less, it is possible to relatively ensure the amount of Zr and improve the barrier property of the plating layer 230.

The dendrite-shaped crystal 231 includes metallic Zn as described above and may additionally include Ni and the like.

For the dendrite-shaped crystals 231, diffraction patterns attributed to the crystal structures can be obtained when electron beam diffraction is carried out on a cross section of the plating layer 230 using a transmission electron microscope (TEM).

The intercrystal filling region 232 is made of, in terms of the average concentration, Zr: 10 to 80 atm %, Zn: 3 to 40 atm %, residual oxygen, and impurities. When the concentration of Zr in the intercrystal filling region 232 is 10 atm % or more, it is possible to enhance the barrier property. In addition, when the concentration of Zr in the intercrystal filling region 232 is 80 atm % or less, it is possible to ensure the barrier property by preventing the generation of cracks in the plating layer 230. In addition, when the concentration of Zn in the intercrystal filling region 232 is 3 atm % or more, it is possible to enhance the barrier property. In addition, when the concentration of Zn in the intercrystal filling region 232 is 40 atm % or less, it is possible to relatively ensure the amount of Zr and improve the barrier property of the plating layer 230.

The amorphous layer 250 is made of, in terms of the average concentration, Zr: 10 to 60 atm %, Zn: 0 to 15 atm %, residual oxygen, and impurities. When the concentration of Zr in the amorphous layer 250 is 10 atm % or more, it is possible to enhance the barrier property. In addition, when the concentration of Zr in the amorphous layer 250 is 60 atm % or less, it is possible to ensure the barrier property by preventing the generation of cracks. The amorphous layer 250 may include a small amount of Zn or no Zn.

Similar to the first embodiment, a base-material layer 220 may be formed between the steel sheet 201 and the plating layer 230.

Similar to the first embodiment, a surface layer 240 may be formed on the plating layer 230 (the amorphous layer 250 when the amorphous layer 250 is formed).

The surface-treated steel sheet 210 of the present embodiment has an L* value, which represent the brightness, of 40 or less and has a black external appearance. The black external appearance makes the surface-treated steel sheet available in a variety of fields. When the L* value exceeds 40, it is difficult to use the steel sheet as a material having a black external appearance. Particularly, when the concentration of Zr in the plating layer 230 is set to 5% by mass or more, it is possible to reliably set to the L* value to 40 or less.

In addition, regarding the surface-treated steel sheet 210 of the present embodiment, an example in which the plating layer 230 is formed on the steel sheet 201 has been described, but the present embodiment is not limited thereto, and the plating layer 230 in the present embodiment may be formed on galvanized layers in electrogalvanized steel sheets, hot-dip galvanized steel sheets, and galvannealed steel sheets. That is, a second galvanized layer (not shown) containing zinc may be further formed between the steel sheet 201 and the plating layer 230. When the second galvanized layer (not shown) is further formed, the corrosion resistance of the surface-treated steel sheet 210 can be further improved. For example, even when corrosive substances pass through the plating layer 230, a sacrificial protection effect can be showed due to the second galvanized layer (not shown), and the corrosion resistance of the surface-treated steel sheet 210 can be improved.

“Method for Manufacturing Surface-Treated Steel Sheet 210”

Next, a method for manufacturing the surface-treated steel sheet 210 will be described. The method for manufacturing the surface-treated steel sheet 210 and the method for manufacturing the surface-treated steel sheet 1 according to the first embodiment are different from each other only in terms of the composition of the plating bath and are the same as each other in terms of the other facts.

Similar to the first embodiment, the method for manufacturing the surface-treated steel sheet 210 has the base-material-forming process and the upper layer plating process. In the base-material-forming process and the upper layer plating process, the same plating bath is used, and a plating bath including a Zr compound (ZrO²⁺) and a Zn compound (Zn²⁺) is used.

The Zr compound is preferably a compound that forms ZrO²⁺ ions in the plating bath, and examples thereof include soluble salts such as zirconium nitrate oxide, zirconium sulfate oxide, and zirconium nitrate chloride oxide. These Zr compounds may be used singly or two or more Zr compounds may be jointly used.

The plating bath preferably includes 0.10 to 4.00 mol/l of Zn²⁺ and more preferably includes 0.50 to 2.00 mol/l of Zn²⁺. In addition, the plating bath preferably includes 0.10 to 4.00 mol/l of ZrO²⁺ and more preferably includes 0.50 to 2.00 mol/l of ZrO²⁺. When a plating bath including ZrO²⁺ in the above-described range is used, it is possible to easily form the plating layer 230 having a high amount of Zr and an excellent barrier property. When the amount of ZrO²⁺ in the plating bath is below the above-described range, it becomes difficult to ensure the amount of Zr in the plating layer 230. In addition, when the amount of ZrO²⁺ in the plating bath is above the above-described range, a large amount of Zr is used in the plating bath 2, which becomes economically disadvantageous.

To the plating bath, in addition to the Zr compound and the Zn compound, a pH adjuster, metal compounds other than the Zr compound and the Zn compound, additives, and the like may be added as necessary.

The current densities in the base-material-forming process and the upper layer plating process are the same as those in the first embodiment and thus will not be described.

OTHER EXAMPLES

The present invention is not limited to the above-described embodiments.

The present embodiment has been described using a case in which the plating layers are formed on both surfaces of the steel sheet as an example, but the plating layer may be formed only on one surface of the steel sheet.

In addition, it is preferable that the base-material layer is formed between the steel sheet and the plating layer, but the base-material layer may not be formed. In addition, when the plating layers are formed on both surfaces of the steel sheet, the base-material layer may be formed only between one surface of the steel sheet and the plating layer.

In the present embodiment, a case in which the plating layer includes vanadium and a case in which the plating layer includes zirconium have been separately described, but these embodiments may be included at the same time.

In addition, the present embodiment has been described using a case in which the surface layer is formed on the surface of the plating layer as an example, but the surface layer may not be formed. The surface-treated steel sheet of the present embodiment has an excellent barrier property, and thus the surface layer for improving the barrier property may not be formed on the surface of the plating layer. In addition, when the plating layers are formed on both surfaces of the steel sheet, the surface layer may be formed on the surface of the plating layer only on one surface.

In addition, the present embodiment has been described using a case in which the surface-treated steel sheet is manufactured using the plating apparatus shown in FIG. 3 as an example, but the plating apparatus for manufacturing the surface-treated steel sheet is not limited to the plating apparatus shown in FIG. 3. For example, in the plating apparatus shown in FIG. 3, four positive electrodes 3 are disposed, but the number of the positive electrodes 3 is not limited. In addition, the sizes and shapes of the plating tank 21, the steel sheet 1, and the positive electrode 3, the dispositions and shapes of the upper portion supply pipe 2 a and the lower portion supply pipe 2 b are not particularly limited and can be appropriately determined depending on the applications and the like of the surface-treated steel sheet 10.

Example 1

“Test Results of Vanadium-Containing Surface-Treated Steel Sheet”

A surface-treated steel sheet having plating layers including vanadium on both surfaces of a steel sheet was produced using the plating apparatus shown in FIG. 3 and a method described below and was evaluated.

A plating bath in a fluid state was prepared by circulating a plating bath having a plating bath composition, a temperature, and a pH shown in Table 1 at a relative average flow rate of 100 m/min.

TABLE 1 Plating bath composition (mol/l) Plating bath V(V⁴⁺) + temperature Plating Zn²⁺ (VO²⁻) Na⁺ Ni²⁻ (° C.) bath pH Bath (Zn—V) 1.0 0.8 1.3 0.1 50 2.2 Bath (Zn) 1.0 — — — 50 1.0

As the steel sheet, a 0.5 mm-thick SPCD steel sheet which is a drawing quality cold-rolled steel sheet defined by JIS G 3141 was used.

A pretreatment (nickel plating) was carried out on the steel sheet, and the steel sheet was used as a negative electrode.

In the pretreatment, first, as a plating bath for the nickel plating, ion exchange water, dense sulfuric acid, and NiSO₄.6H₂O were mixed together, thereby adjusting a plating bath having a concentration of Ni²⁺ of 60 g/L and a pH at 60° C. of 2.0. In addition, the steel sheet was immersed in the plating bath, and an electrolytic treatment was carried out using the steel sheet as a negative electrode and a platinum electrode as a positive electrode so that the adhered amount of Ni reached 200 mg/m².

In a base-material-forming process and an upper layer plating process, individual electric conduction times were set to times shown in Table 2 and Table 3, and a plating layer was formed using an electroplating method.

TABLE 2 Base-material- Upper layer forming process plating process Current Current density density Example Plating bath (Seconds) A/dm² (Seconds) A/dm² V1 Bath (Zn—V) 8 10 3 100 V2 Bath (Zn—V) 4 10 3 100 V3 Bath (Zn—V) 2 10 3 100 V4 Bath (Zn—V) 1 10 3 100 V5 Bath (Zn—V) 0.5 10 3 100 V6 Bath (Zn—V) 0.2 10 3 100 V7 Bath (Zn—V) 0.05 10 3 100 V8 Bath (Zn—V) 1 18 3 100 V9 Bath (Zn—V) 1 5 3 100 VI 0 Bath (Zn—V) 4 18 3 100 V11 Bath (Zn—V) 4 14 3 100 V12 Bath (Zn—V) 4 5 3 100 V13 Bath (Zn—V) 4 0 3 100 V14 Bath (Zn—V) 1 10 6 100 V15 Bath (Zn—V) 1 10 12 100 V16 Bath (Zn—V) 1 10 3 150 V17 Bath (Zn—V) 1 10 3 50 V18 Bath (Zn—V) 1 10 3 100 V19 Bath (Zn—V) 1 10 3 100 V20 Bath (Zn—V) 1 10 3 100

TABLE 3 Base-material- Upper layer forming process plating process Current Current Comparative density density Example Plating bath (Seconds) A/dm² (Seconds) A/dm² X1 Bath (Zn—V) 0 — 15 21 X2 Bath (Zn—V) 4 25 3 100 X3 Bath (Zn—V) 1 25 3 100 X4 Bath (Zn—V) 0 — 18 18 X5 Bath (Zn—V) 0 — 3 100 X6 Bath (Zn) 1 10 12 100 X7 Bath (Zn—V) 12 10 3 100

Meanwhile, in the plating bath composition shown in Table 1, ZnSO₄.7H₂O was used as a Zn compound, VOSO₄.5H₂O was used as a V compound, and furthermore, Na₂SO₄ and, as another metal compound, NiSO₄.6H₂O were used as necessary. The amounts thereof were adjusted so as to obtain the concentrations of Zn²⁺, V (V⁴⁺, VO²⁺), Na⁺, and Ni²⁺ shown in Table 1.

The plating layers of examples and comparative examples obtained as described above were observed using a field-emission transmission electron microscope (FE-TEM) (manufactured by JEOL Ltd. (JED-2100F)).

FIG. 5A to FIG. 5C are the transmission electron microscopic (TEM) photographs of a plating layer in a surface-treated steel sheet of Example V4. FIG. 5A is a cross sectional photograph in the entire thickness direction of the plating layer 30 formed on the steel sheet 1, FIG. 5B is an enlarged photograph of an interface portion between the steel sheet and the plating layer in the cross section of FIG. 5A, and FIG. 5C is an enlarged photograph of dendrite-shaped crystals and peripheral portions thereof in the cross section of FIG. 5A.

In FIG. 5B, a reference symbol 51 indicates a base-material layer, and a reference symbol 52 indicates an intercrystal filling region in the vicinity of an interface between the steel sheet and the plating layer. In addition, in FIG. 5C, a reference symbol 53 indicates a dendrite-shaped crystal, a reference symbol 54 indicates an intercrystal filling region, and a reference symbol 55 indicates a surface layer formed on the surface of the dendrite-shaped crystal.

As shown in FIG. 5A to FIG. 5C, in the surface-treated steel sheet of Example V4, dendrite-shaped crystals, intercrystal filling regions, and the surface layers of the dendrite-shaped crystals were formed in the plating layer.

Similar to Example V4, the plating layers in the surface-treated steel sheets of Examples V1 to V3 and V5 to V20 were observed using TEM. As a result, dendrite-shaped crystals, intercrystal filling regions, and the surface layers of the dendrite-shaped crystals were formed in the plating layers.

The plating layer in the surface-treated steel sheet of Example V4 was observed using a scanning electron microscope (SEM: A-4300SE manufactured by Hitachi, Ltd.) in a cross section direction. The plating layer was observed after a gold film was deposited on the surface of the plating layer in order to facilitate the observation of the surface shape of the plating layer.

FIG. 6 is a scanning electron microscopic (SEM) photograph of the plating layer in the surface-treated steel sheet of Example V4. In FIG. 6, a reference symbol 56 indicates a dendrite-shaped crystal, a reference symbol 57 indicates an intercrystal filling region disposed between dendrite-shaped crystals, and a reference symbol 58 indicates a surface layer covering the surface of the dendrite-shaped crystal. Meanwhile, in the photograph shown in FIG. 6, white portions on the surface of the plating layer are the gold film deposited to observe the plating layer.

Similar to the plating layer in Example V4, in the plating layers in the surface-treated steel sheets of Examples V1 to V3 and V5 to V20, dendrite-shaped crystals, intercrystal filling regions, and the surface layers of the dendrite-shaped crystals were formed.

Similar to Example V4, the plating layers in the surface-treated steel sheets of Comparative Example x1 to Comparative Example x7 were observed using SEM.

FIG. 7 is a scanning electron microscopic (SEM) photograph of the plating layer in the surface-treated steel sheet of Comparative Example x2. As shown in FIG. 7, the plating layer in Comparative Examples x2 was a single phase made of a dendrite-shaped crystal.

For the plating layers in Examples V1 to Example V20, elements in the dendrite-shaped crystals, the intercrystal filling regions, and the surface layers of the dendrite-shaped crystals were each analyzed using an energy dispersive X-ray analyzer (EDS) (manufactured by JEOL (JED-2300T)). In addition, elements (composition) in the dendrite-shaped crystals, elements (composition) in the intercrystal filling regions, the amount of vanadium, the amount of zinc, and elements (composition) in the surface layers of the dendrite-shaped crystals were investigated.

In addition, the molar ratio (V/Zn) of the amount of vanadium to the amount of zinc in the intercrystal filling regions was computed using the results of the element analyses.

For the plating layers in Comparative Example x1 to Comparative Example x7 as well, elements in the dendrite-shaped crystals, the intercrystal filling regions, and the surface layers of the dendrite-shaped crystals were analyzed in the same manner as in Example V1 to Example V20.

The results are shown in Table 4 and Table 5.

TABLE 4 (C) Surface layer of (A) Dendrite-shaped crystal (B) Intercrystal filling region dendrite-shaped crystal Lower layer plating Presence Presence V/Zn Presence Presence Adhered or or molar or or amount Example absence Composition absence Composition ratio absence Composition absence Composition (g/m²) V1 Present Zn (hexagonal) Present Zn, V, O (amorphous) 0.20 Present ZnO (crystal) Absent — — V2 Present Zn (hexagonal) Present Zn, V, O (amorphous) 1.20 Present ZnO (crystal) Absent — — V3 Present Zn (hexagonal) Present Zn, V, O (amorphous) 0.80 Present ZnO (crystal) Absent — — V4 Present Zn (hexagonal) Present Zn, V, O (amorphous) 0.60 Present ZnO (crystal) Absent — — V5 Present Zn (hexagonal) Present Zn, V, O (amorphous) 0.40 Present ZnO (crystal) Absent — — V6 Present Zn (hexagonal) Present Zn, V, O (amorphous) 0.30 Present ZnO (crystal) Absent — — V7 Present Zn (hexagonal) Present Zn, V, O (amorphous) 0.20 Present ZnO (crystal) Absent — — V8 Present Zn (hexagonal) Present Zn, V, O (amorphous) 0.70 Present ZnO (crystal) Absent — — V9 Present Zn (hexagonal) Present Zn, V, O (amorphous) 0.50 Present ZnO (crystal) Absent — — V10 Present Zn (hexagonal) Present Zn, V, O (amorphous) 0.30 Present ZnO (crystal) Absent — — V11 Present Zn (hexagonal) Present Zn, V, O (amorphous) 0.80 Present ZnO (crystal) Absent — — V12 Present Zn (hexagonal) Present Zn, V, O (amorphous) 0.90 Present ZnO (crystal) Absent — — V13 Present Zn (hexagonal) Present Zn, V, O (amorphous) 0.15 Present ZnO (crystal) Absent — — V14 Present Zn (hexagonal) Present Zn, V, O (amorphous) 0.70 Present ZnO (crystal) Absent — — V15 Present Zn (hexagonal) Present Zn, V, O (amorphous) 0.80 Present ZnO (crystal) Absent — — V16 Present Zn (hexagonal) Present Zn, V, O (amorphous) 0.70 Present ZnO (crystal) Absent — — V17 Present Zn (hexagonal) Present Zn, V, O (amorphous) 0.40 Present ZnO (crystal) Absent — — V18 Present Zn (hexagonal) Present Zn, V, O (amorphous) 0.60 Present ZnO (crystal) Present Zn 3 V19 Present Zn (hexagonal) Present Zn, V, O (amorphous) 0.60 Present ZnO (crystal) Present Zn 10 V20 Present Zn (hexagonal) Present Zn, V, O (amorphous) 0.60 Present ZnO (crystal) Present Zn 20

TABLE 5 (C) Surface layer of (A) Dendrite-shaped crystal (B) Intercrystal filling region dendrite-shaped crystal Lower layer plating Presence Presence Presence Presence Adhered Comparative or or V/Zn molar or or amount Example absence Composition absence Composition ratio absence Composition absence Composition (g/m²) X1 Present Zn (hexagonal) Present Unstable 0.02 Absent — Absent — — X2 Present Zn (hexagonal) Present Unstable 0.04 Absent — Absent — — X3 Present Zn (hexagonal) Present Unstable 0.06 Absent — Absent — — X4 Absent Zn (hexagonal) Absent — — Absent — Absent — — X5 Present Zn (hexagonal) Present Unstable 0.08 Present ZnO (crystal) Absent — — X6 Absent Zn (hexagonal) Absent — — Absent — Absent — — X7 Absent Zn (hexagonal) Absent Zn, V, O 0.08 Present ZnO (crystal) Absent — — (amorphous)

In addition, for the plating layers in the surface-treated steel sheets of Examples V1 to V20, whether the dendrite-shaped crystals, the intercrystal filling regions, and the surface layers of the dendrite-shaped crystals respectively had crystal structures or were amorphous was checked using electron beam diffraction images obtained using TEM in the cross section direction.

FIG. 8 is a photograph showing electron beam diffraction images of the plating layer in Example V4. The reference symbols in the photograph shown in FIG. 8 respectively correspond to the intercrystal filling region 52, the dendrite-shaped crystal 53, the intercrystal filling region 54, and the surface layer 55 of the dendrite-shaped crystal shown in FIG. 5B and FIG. 5C.

From the electron beam diffraction images shown in FIG. 8, it was found that the dendrite-shaped crystal 53 and the surface layer 55 of the dendrite-shaped crystal 53 had crystal structures. In addition, it was found that, for the intercrystal filling region 52 and the intercrystal filling region 54, diffraction patterns attributed to crystal structures could not be obtained, and thus they were amorphous.

For the plating layers in the surface-treated steel sheets of Examples V1 to V3 and V5 to V20, whether the dendrite-shaped crystals, the intercrystal filling regions, and the surface layers of the dendrite-shaped crystals respectively had crystal structures or were amorphous was checked in the same manner as in Example V4. As a result, it was found that the dendrite-shaped crystals and the surface layers of the dendrite-shaped crystals had crystal structures, and the intercrystal filling regions were amorphous.

Meanwhile, impurities in the dendrite-shaped crystals were investigated, and it was found that the amounts of C, Si, S, Fe, and N were respectively approximately 0.1 to 5 atm %.

The plating layers in the surface-treated steel sheets of Comparative Example x1 to Comparative Example x7 were analyzed using an X-ray diffraction apparatus (XRD: R1NT 2500 manufactured by Rigaku Corporation). As a result, for the plating layers in Comparative Example x1 to Comparative Example x7, it was confirmed that the dendrite-shaped crystals had Zn crystal structures. In addition, except for Comparative Example x7, cases in which the intercrystal filling regions were not formed (Comparative Examples x4 and x6) or cases in which only unstable amorphous diffraction patterns could be obtained even when the intercrystal filling regions were formed (Comparative Examples x1 to x3 and x5) were confirmed.

In addition, for the plating layers in the examples and the comparative examples, the following items were evaluated using methods described below.

(Adhered Amount in Plating Layer and Content Ratio of Vanadium)

As the adhered amount of the plating layer, the total mass of a Zn element and a V element per unit area which were detected using a fluorescent X-ray apparatus (Simultix 14 manufactured by Rigaku Corporation) was used. In addition, the amount of vanadium in the plating layer was computed as a percentage by dividing the amount of the V element detected using the fluorescent X-ray apparatus by the adhered amount.

“Barrier Property”

The edge and rear surface of a test specimen cut out from the surface-treated steel sheet were sealed with tape, and the method of salt spray testing (JIS-Z-2371) was carried out. In addition, the area ratio of white rust generation in the non-sealed portions was visually observed after 72 hours and was evaluated using the following standards. Meanwhile, the area ratio of white rust generation is the percentage of the area of white rust generation portions in the area of the observation portion.

(Standards)

5: The area ratio of white rust generation is less than 10%

4: The area ratio of white rust generation is 10% or more and less than 25%

3: The area ratio of white rust generation is 25% or more and less than 50%

2: The area ratio of white rust generation is 50% or more and less than 75%

1: The area ratio of white rust generation is 75% or more

“Sacrificial Protection Property”

Coating (manufactured by Kansai Paint Co., Ltd., AMIRAC #1000) was applied to a test specimen cut out from the surface-treated steel sheet by means of bar coating and was baked at 140° C. for 20 minutes, thereby forming a film having a dried film thickness of 25 μm. After that, the edge and rear surface of the coated test specimen were sealed with tape, and an X-shaped mark was provided on the surface using an NT cutter. In addition, the method of salt spray testing (JIS-Z-2371) was carried out, and the time taken for red rust to be generated from the mark portion was measured and evaluated using the following standards.

(Standards)

5: The time taken for red rust to be generated is 960 hours or longer

4: The time taken for red rust to be generated is 720 hours or longer and shorter than 960 hours

3: The time taken for red rust to be generated is 480 hours or longer and shorter than 720 hours

2: The time taken for red rust to be generated is 120 hours or longer and shorter than 480 hours

1: The time taken for red rust to be generated is shorter than 120 hours

(Powdering Property (Adhesion between Plating Layer and Steel Sheet))

In the powdering test, a 60° V-bent die was used. A test specimen cut out from the surface-treated steel sheet was bent at 60° using a die having a curvature radius of 1 mm at the front end so that the evaluation surface of the test specimen became the inside of a bent portion, tape was attached to the inside of the bent portion, and the tape was peeled off. The powdering property (peeled width (mm)) was evaluated from the peeling status of the plating layer peeled together with the tape.

(Coating Film Adhesion)

Coating (manufactured by Kansai Paint Co., Ltd., AMIRAC #1000) was applied to a test specimen cut out from the surface-treated steel sheet by means of bar coating and was baked at 140° C. for 20 minutes, thereby forming a film having a dried film thickness of 25 μm. The obtained coated sheet was immersed in boiling water for 30 minutes and then left to stand for 24 hours indoors at normal temperature. After that, 100 1 mm×1 mm grids were provided on the test specimen using an NT cutter, pushed out 7 mm using an Erichsen distensibility tester, then, a peeling test was carried out on pushed-out protrusion portions using adhesive tape, and the coating film adhesion (the number of peeled portions) was evaluated.

TABLE 6 Plating Corrosion resistance Coating film Adhered V content Sacrificial Powdering adhesion amount ratio Barrier protection property Number of Example g/m² % by mass property property (mm) peeled portions V1 5.0 3 3 3 0 0 V2 5.0 16 5 1 5 0 V3 5.0 11 5 2 2 0 V4 5.0 8 5 2 1 0 V5 5.0 5 4 2 2 0 V6 5.0 4 4 2 0 0 V7 5.0 3 3 2 0 0 V8 5.0 9 5 2 1 0 V9 5.0 7 4 2 1 0 V10 5.0 4 4 2 0 0 V11 5.0 11 5 2 1 0 V12 5.0 12 5 2 2 0 V13 5.0 2 2 3 0 0 V14 10.0 9 5 3 1 0 V15 20.0 11 5 4 2 0 V16 7.5 9 5 3 1 0 V17 2.5 5 4 1 0 0 V18 5.0 8 5 4 1 0 V19 5.0 8 5 5 1 0 V20 5.0 8 5 5 1 0

TABLE 7 Plating Corrosion resistance Coating film Adhered V content Sacrificial Powdering adhesion Comparative amount ratio Barrier protection property Number of Example g/m² % by mass property property (mm) peeled portions X1 5 1 1 4 0 100 X2 5 1 1 3 0 100 X3 5 2 2 3 0 70 X4 5 0.5 1 5 0 100 X5 5 8 2 2 2 0 X6 20 0 1 5 0 100 X7 5 1 1 3 0 0

As shown in Table 6 and Table 7, it was found that the surface-treated steel sheets of Example V1 to Example V20 satisfied all of the scopes of the present invention and had a superior barrier property and superior coating film adhesion compared with the surface-treated steel sheets of Comparative Example x1 to Comparative Example x7.

In addition, in the plating layers in the surface-treated steel sheets of Example V1 to Example V20 in which the current densities were 0 to 18 A/dm² in the base-material-forming process, unlike the plating layers in the surface-treated steel sheets of Comparative Examples x2 and x3 in which the current densities were 25 A/dm² in the base-material-forming process, the molar ratio (V/Zn) of vanadium to zinc in the intercrystal filling regions reached 0.10 or more and 2.00 or less, and the intercrystal filling regions showed amorphous diffraction patterns when electron beam diffraction was carried out. In addition, it was found that the barrier property and the coating film adhesion were superior.

Example 2

“Test Results of Film-Forming Vanadium-Containing Surface-Treated Steel Sheet”

A coating composition for forming films was prepared by stirring and dispersing the organic resin (R), the phosphoric acid compound (P), the carbon black (C), the organic silicon compound (W), the fluoro metal complex compound (F), the isocyanate compound (I), and the polyethylene wax (Q) shown in Table 8 in water which was a solvent at amounts (% by mass of the solid contents) shown in Table 9 and Table 10 using a coating disperser.

TABLE 8 Reference Kind Symbol Names of compounds and the like Organic resin R Polyurethane resin (SUPERFLEX620 manufactured by DKS Co., Ltd.) Phosphoric acid compound P1 Phosphoric acid P2 Ammonium dihydrogen phosphate Carbon black C EMF BLACK HK-3 manufactured by Toyocolor Co., Ltd. Organic silicon compound W1 3-Aminopropyltriethoxysilane W2 3-Glycidoxypropyltrimethoxysilane Fluoro metal complex compound F Hexafluorotitanic acid Isocyanate compound I Blocked isocyanate (Aqua BI220 manufactured by Baxenden Chemicals Limited), dissociation temperature 120° C. Polyethylene wax Q Polyethylene resin particles (CHEMIPEARL W950 manufactured by Mitsui Chemicals, Inc., particle diameters 0.6 μm)

TABLE 9 Phosphoric Fluoro acid Organic silicon metal Organic compound Concentration Carbon compound (W) complex Isocyanate resin (P) of phosphoric black (W1)/ compound compound Polyethylene R/ Coating Number Plating (R) P1 P2 acid ions (C) (W1) (W2) (W2) (F) (I) wax (Q) W stability Example t1 V4 39 6 — — 0 25 25 1.0 — — 5 0.8 OK Example t2 V4 37 6 4  9.3 0 24 24 1.0 — — 5 0.8 OK Example t3 V4 35 6 9 13.5 0 23 22 1.0 — — 5 0.8 OK Example t4 V4 33 6 9 13.5 5 21 21 1.0 — — 5 0.8 OK Example t5 V4 30 6 9 13.5 10 20 20 1.0 — — 5 0.8 OK Example t6 V4 28 6 9 13.5 15 19 18 1.1 — — 5 0.8 OK Example t7 V4 25 6 9 13.5 20 18 17 1.1 — — 5 0.7 OK Example t8 V4 45 6 9 13.5 10 13 12 1.1 — — 5 1.8 OK Example t9 V4 50 6 9 13.5 10 10 10 1.0 — — 5 2.5 OK Example t10 V4 40 6 9 13.5 10 10 10 1.0 — 10 5 2.0 OK Example t11 V4 40 6 9 13.5 10 8 7 1.1 5 10 5 2.7 OK Example t12 V4 35 6 9 13.5 10 8 7 1.1 5 15 5 2.3 OK Example t13 V4 35 6 9 13.5 10 8 7 1.1 5 15 5 2.3 OK Example t14 V4 35 6 9 13.5 10 8 7 1.1 5 15 5 2.3 OK

TABLE 10 Phosphoric Fluoro acid Organic silicon metal Organic compound Concentration Carbon compound (W) complex Isocyanate Poly- resin (P) of phosphoric black (W1)/ compound compound ethylene Coating Number Plating (R) P1 P2 acid ions (C) (W1) (W2) (W2) (F) (I) wax (Q) R/W stability Comparative w1 x3 35 6 9 13.5 0 23 22 1.0 — — 5 0.8 OK Example Comparative w2 x3 45 6 9 13.5 10 13 12 1.1 — — 5 1.8 OK Example Comparative w3 x3 40 6 9 13.5 10 10 10 1.0 — 10 5 2.0 OK Example Comparative w4 x3 40 6 9 13.5 10 9 8 1.1 3 10 5 2.4 OK Example Comparative w5 x3 40 6 9 13.5 10 9 8 1.1 — 10 5 2.4 OK Example Reference e1 — 38 9 — 9 0 24 24 1.0 — — 5 0.8 NG Example Reference e2 V4 No film Example

In the adjustment of the coating composition, an aqueous liquid including a hydrolysis condensate generated by dissolving 3-aminopropyl triethoxysilane (W1) and 3-glycidoxypropyl trimethoxysilane (W2) shown in Table 8 in water at a ratio {(W1)/(W2)} shown in Table 9 and Table 10 was added thereto as the organic silicon compound (W) so as to obtain the amounts shown in Table 9 and Table 10.

(Coating Stability)

The coating composition prepared as described above was stirred at room temperature for 30 minutes, and whether or not sediment was generated was visually observed.

Coating compositions in which sediment was not generated were evaluated as “OK” in terms of the coating stability, and coating compositions in which sediment was generated were evaluated as “NG” in terms of the coating stability. The results of the coating stability are shown in Table 9 and Table 10.

As shown in Table 9 and Table 10, the coating compositions that were used in Examples t1 to t14 and Comparative Examples w1 to w5 were evaluated as “OK” in terms of the coating stability and had excellent stability.

Next, films were formed on the surface of the plating layer in the surface-treated steel sheet of Example V4 or Comparative Example x3 manufactured in (Example I) respectively using the coating compositions and a method described below.

First, the coating compositions were applied to the surface of the surface-treated steel sheet using a roll coater so as to obtain film thicknesses shown in Table 11 and Table 12. After that, the surface-treated steel sheet to which the coating compositions had been applied was heated and dried so that the sheet reaching temperature reached 150° C. and was spray-cooled using water, thereby obtaining films. Meanwhile, even after the heating to 150° C., hydrated oxides were present in the plating layers.

Next, for the respective surface-treated steel sheets having the film formed on the surface of the plating layer, the external appearance evenness, the corrosion resistance, the electric conductivity, the working adhesion, and the scratch resistance were respectively evaluated. In addition, as Reference Example e2, the external appearance evenness, corrosion resistance, electric conductivity, working adhesion, and scratch resistance of the surface-treated steel sheet of Example V4 manufactured in (Example 1) were evaluated.

The evaluation results of the respective items are shown in Table 11 and Table 12.

TABLE 11 Film After forming of film thickness Appearance Corrosion Electric Working Scratch Number μm evenness resistance conductivity adhesion resistance Example t1 1 2 4 5 5 2 Example t2 1 2 5 5 5 2 Example t3 1 2 6 5 5 2 Example t4 1 3 5 5 5 2 Example t5 1 4 5 5 5 2 Example t6 1 4 4 5 5 2 Example t7 1 4 4 5 5 2 Example t8 1 4 5 5 5 3 Example t9 1 4 5 5 5 3 Example t10 1 4 5 5 5 3 Example t11 1 4 5 5 5 5 Example t12 1 4 5 5 5 5 Example t13 3 4 6 4 5 5 Example t14 5 4 6 1 5 4

TABLE 12 Film After forming of film thickness Appearance Corrosion Electric Working Scratch Number μm evenness resistance conductivity adhesion resistance Comparative w1 1 1 1 5 1 1 Example Comparative w2 1 1 1 5 1 1 Example Comparative w3 1 1 1 5 1 1 Example Comparative w4 1 1 1 5 1 1 Example Comparative w5 1 1 1 5 1 1 Example Reference e1 — — — — — — Example Reference e2 — 2 1 6 — — Example

The evaluation methods and evaluation standards of the respective items will be described below.

(External Appearance Evenness)

The L* value of the surface-treated steel sheet was measured using a colorimeter manufactured by Konica Minolta Japan, Inc.: CR-400 and was evaluated using the following evaluation standards.

(Evaluation Standards)

5: The L* value is less than 24

4: The L* value is 24 or more and less than 27

3: The L* value is 27 or more and less than 28

2: The L* value is 28 or more and less than 30

1: The L* value is more than 30

(Corrosion Resistance)

The edge and rear surface of a test specimen cut out from the surface-treated steel sheet were sealed with tape, and salt spray testing (JIS-Z-2371) was carried out. In addition, the area ratio of white rust generation in the non-sealed portions was visually observed after 240 hours and was evaluated using the following standards. The area ratio of white rust generation is the percentage of the area of white rust generation portions in the area of the observation portion.

(Evaluation Standards)

6: The ratio of white rust generation is less than 3%

5: The ratio of white rust generation is 3% or more and less than 10%

4: The ratio of white rust generation is 10% or more and less than 25%

3: The ratio of white rust generation is 25% or more and less than 50%

2: The ratio of white rust generation is 50% or more and less than 75%

1: The ratio of white rust generation is 75% or more

(Electric Conductivity)

The interlayer resistance value (Ω·cm²) was measured using a test specimen cut out from the surface-treated steel sheet and the measurement method defined in JIS C 2550, and the electric conductivity was evaluated using the following standards.

(Evaluation Standards)

6: The interlayer resistance value is less than 1.0 Ω·cm²

5: The interlayer resistance value is 1.0 Ω·cm² or more and less than 1.5 Ω·cm²

4: The interlayer resistance value is 1.5 Ω·cm² or more and less than 2.0 Ω·cm²

3: The interlayer resistance value is 2.0 Ω·cm² or more and less than 2.5 Ω·cm²

2: The interlayer resistance value is 2.5 Ω·cm² or more and less than 3.0 Ω·cm²

1: The interlayer resistance value is 3.0 Ω·cm² or more

(Working Adhesion)

A test specimen cut out from the surface-treated steel sheet was bent 180°, and a tape peeling test was carried out on the outside of the bent portion. The external appearance of the tape-peeled portion was observed using a magnifying glass having an enlargement factor of ten times and was evaluated using the following standards. The test specimen was bent in an atmosphere of 20° C. with a 0.5 mm spacer sandwiched therebetween.

(Evaluation Standards)

5: The peeling of the coating film is not observed

4: The peeling of an extremely small part of the coating film is observed (peeling area≤2%)

3: The peeling of a part of the coating film is observed (2%<peeling area≤10%)

2: The peeling of the coating film is observed (10%<peeling area≤20%)

1: The peeling of the coating film is observed (peeling area>20%)

(Scratch Resistance)

A material under test cut out from the surface-treated steel sheet was adhered to the electrogalvanized steel sheet (non-treated material), and the material under test was rotated 90° in a pressed state. The applied pressure was set to 0.2 kg/cm², and the testing temperature was set to 25° C. After that, the external appearance of the material under test was visually evaluated.

(Evaluation standards)

6: Scratches are not visible

5: There are fine scratches, but the base material is not exposed

4: The base material is slightly exposed (exposed area: less than 3%)

3: The base material is exposed (exposed area: 3% or more and less than 10%)

2: The base material is exposed (exposed area: 10% or more and less than 30%)

1: The base material is exposed (exposed area: 30% or more)

As shown in Table 11 and Table 12, in Examples t1 to t14 in which the film was provided on the surface of the plating layer in the surface-treated steel sheet of Example V4, the evaluation results were 2 or higher in terms of the evaluation standards in all of the evaluation items, and excellent external appearance evenness, corrosion resistance, electric conductivity, working adhesion, and scratch resistance were showed. In addition, compared with Reference Example e2 in which no film was formed on the surface, Examples t1 to t14 had favorable corrosion resistance and favorable electric conductivity.

Meanwhile, in Comparative Examples w1 to w5 in which the film was provided on the surface of the plating layer in the surface-treated steel sheet of Comparative Example x3, the evaluation results were 1 in terms of the external appearance evenness, the corrosion resistance after 240 hours, the working adhesion, and the scratch resistance, and the performance was poor.

Example 3

“Test Results of Zirconium-Containing Surface-Treated Steel Sheet”

A surface-treated steel sheet having plating layers including zirconium on both surfaces of a steel sheet was produced using the plating apparatus shown in FIG. 2 and a method described below and was evaluated.

Meanwhile, the same facts as in Example 1 will not be described.

A plating bath in a fluid state was prepared by circulating a plating bath having a plating bath composition, a temperature, and a pH shown in Table 13 at a relative average flow rate of 100 m/min.

TABLE 13 Plating bath composition (mol/l) Plating bath Zr(Zr⁴⁺) + temperature Plating Zn²⁺ (ZrO²⁺) Na⁺ Ni²⁻ (° C.) bath pH Bath (Zn—Zr) 1.0 0.5 1.3 0.1 50 1.5 Bath (Zn) 1.0 — — — 50 1.0

In addition, a pretreatment (nickel plating) was carried out using the steel sheet as a negative electrode. In the base-material-forming process and the upper layer plating process, the electric conduction times were set to times shown in Table 14 and Table 15, the current densities were set to numerical values shown in Table 14 and Table 15, and a plating layer was formed using an electroplating method.

TABLE 14 Base-material- Upper layer forming process plating process Current Current density density Example Plating bath (Seconds) A/dm² (Seconds) A/dm² Z1 Bath (Zn—Zr) 8 10 6 50 Z2 Bath (Zn—Zr) 4 10 6 50 Z3 Bath (Zn—Zr) 2 10 6 50 Z4 Bath (Zn—Zr) 1 10 6 50 Z5 Bath (Zn—Zr) 0.5 10 6 50 Z6 Bath (Zn—Zr) 0.2 10 6 50 Z7 Bath (Zn—Zr) 0.05 10 6 50 Z8 Bath (Zn—Zr) 1 18 6 50 Z9 Bath (Zn—Zr) 1 5 6 50 Z10 Bath (Zn—Zr) 4 18 6 50 Z11 Bath (Zn—Zr) 4 14 6 50 Z12 Bath (Zn—Zr) 4 5 6 50 Z13 Bath (Zn—Zr) 4 0 6 50 Z14 Bath (Zn—Zr) 1 10 12 50 Z15 Bath (Zn—Zr) 1 10 24 50 Z16 Bath (Zn—Zr) 1 10 6 75 Z17 Bath (Zn—Zr) 1 10 6 25 Z18 Bath (Zn—Zr) 1 10 6 50 Z19 Bath (Zn—Zr) 1 10 6 50 Z20 Bath (Zn—Zr) 1 10 6 50

TABLE 15 Base-material- Upper layer forming process plating process Current Current Comparative density density Example Plating bath (Seconds) A/dm² (Seconds) A/dm² x11 Bath (Zn—Zr) 0 — 6 50 x12 Bath (Zn—Zr) 4 25 6 50 x13 Bath (Zn—Zr) 1 25 6 50 x14 Bath (Zn—Zr) 0 — 6 50 x15 Bath (Zn) 1 10 12 100 x16 Bath (Zn—Zr) 12 10 6 50

Meanwhile, in the plating bath composition shown in Table 13, ZnSO₄.7H₂O was used as the Zn compound, a ZrO(NO₃)₂ aqueous solution or a ZrOSO₄ aqueous solution was used as a Zr compound, and furthermore, Na₂SO₄ and, as another metal compound, NiSO₄.6H₂O were used as necessary. The amounts thereof were adjusted so as to obtain the concentrations of Zn²⁺, Zr (Zr⁴⁺, ZrO²⁺), Na⁺, and Ni²⁺ shown in Table 1.

The plating layers of examples and comparative examples obtained as described above were observed using a field-emission transmission electron microscope (FE-TEM) (manufactured by JEOL Ltd. (JED-2100F)).

FIG. 9 is a transmission electron microscopic (TEM) photograph of the plating layer in the surface-treated steel sheet of Example Z4.

As shown in FIG. 9, it is found that, in the plating layer in the present embodiment, dendrite-shaped crystals, intercrystal filling regions formed in the peripheries thereof, and amorphous layers in surface layer portions were formed. As a result of element analyses and electron beam diffraction analyses using an energy dispersive X-ray analyzer (EDS) (manufactured by JEOL (JED-2300T)), it was found that the dendrite-shaped crystals were made of metallic Zn. In addition, it was found that the intercrystal filling regions included a zirconium oxide and a zirconium hydroxide. Furthermore, it was found that the amorphous layers in the surface layer portions were made of a zirconium oxide.

In the plating layers in other examples as well, similar to Example Z4, dendrite-shaped crystals, intercrystal filling regions formed in the peripheries thereof, and amorphous layers in surface layer portions were formed. In addition, the amorphous layers in the surface layer portions were removed in some plating layers.

For the plating layers in Examples Z1 to Example Z20, elements in “the dendrite-shaped crystals”, “the intercrystal filling regions”, and “the surface layers of the dendrite-shaped crystals” were respectively analyzed using an energy dispersive X-ray analyzer (EDS) (manufactured by JEOL (JED-2300T)). In addition, elements (composition) in the dendrite-shaped crystals, elements (composition) in the intercrystal filling regions, the amount of zirconium, the amount of zinc, and elements (composition) in the surface layers of the dendrite-shaped crystals were investigated.

In addition, the molar ratio (Zr/Zn) of the amount of zirconium to the amount of zinc in the intercrystal filling regions was computed using the results of the element analyses.

For the plating layers in Comparative Example x11 to Comparative Example x16 as well, elements in “the dendrite-shaped crystals”, “the intercrystal tilling regions”, and “the surface layers of the dendrite-shaped crystals” were analyzed in the same manner as in Example Z1 to Example Z20.

The results are shown in Table 16 and Table 17.

TABLE 16 Lower layer plating (A) Dendrite-shaped crystal (B) Intercrystal filling region Attachment Presence or Presence or Zr/Zn Presence or amount Example absence Composition absence Composition molar ratio absence Composition (g/m²) Z1 Present Zn (hexagonal) Present Zn, Zr, O (amorphous) 1.00 Absent — — Z2 Present Zn (hexagonal) Present Zn, Zr, O (amorphous) 2.50 Absent — — Z3 Present Zn (hexagonal) Present Zn, Zr, O (amorphous) 1.67 Absent — — Z4 Present Zn (hexagonal) Present Zn, Zr, O (amorphous) 1.25 Absent — — Z5 Present Zn (hexagonal) Present Zn, Zr, O (amorphous) 1.20 Absent — — Z6 Present Zn (hexagonal) Present Zn, Zr, O (amorphous) 1.20 Absent — — Z7 Present Zn (hexagonal) Present Zn, Zr, O (amorphous) 1.00 Absent — — Z8 Present Zn (hexagonal) Present Zn, Zr, O (amorphous) 1.80 Absent — — Z9 Present Zn (hexagonal) Present Zn, Zr, O (amorphous) 1.30 Absent — — Z10 Present Zn (hexagonal) Present Zn, Zr, O (amorphous) 1.20 Absent — — Z11 Present Zn (hexagonal) Present Zn, Zr, O (amorphous) 1.67 Absent — — Z12 Present Zn (hexagonal) Present Zn, Zr, O (amorphous) 2.20 Absent — — Z13 Present Zn (hexagonal) Present Zn, Zr, O (amorphous) 1.00 Absent — — Z14 Present Zn (hexagonal) Present Zn, Zr, O (amorphous) 1.46 Absent — — Z15 Present Zn (hexagonal) Present Zn, Zr, O (amorphous) 1.67 Absent — — Z16 Present Zn (hexagonal) Present Zn, Zr, O (amorphous) 1.46 Absent — — Z17 Present Zn (hexagonal) Present Zn, Zr, O (amorphous) 1.30 Absent — — Z18 Present Zn (hexagonal) Present Zn, Zr, O (amorphous) 1.25 Present Zn  3 Z19 Present Zn (hexagonal) Present Zn, Zr, O (amorphous) 1.25 Present Zn 10 Z20 Present Zn (hexagonal) Present Zn, Zr, O (amorphous) 1.25 Present Zn 20

TABLE 17 (A) Dendrite- Lower layer plating shaped crystal (B) Intercrystal filling region Adhered Comparative Presence or Presence or Zr/Zn Presence or amount Example absence Composition absence Composition molar ratio absence Composition (g/m²) x11 Present Zn (hexagonal) Absent Zn, Zr, O (unstable) 0.20 Absent — — x12 Present Zn (hexagonal) Absent Zn, Zr, O (unstable) 0.30 Absent — — x13 Absent Zn (hexagonal) Absent Zn, Zr, O (unstable) 0.40 Absent — — x14 Present Zn (hexagonal) Absent Zn, Zr, O (unstable) 0.20 Absent — — x15 Absent Zn (hexagonal) Absent — — Absent — — x16 Present Zn (hexagonal) Present Zn, Zr, O (unstable) 0.50 Absent — —

For the plating layers in the surface-treated steel sheets of Examples Z1 to Z20, whether “the dendrite-shaped crystals”, “the intercrystal filling regions”, and “the surface layers of the dendrite-shaped crystals” respectively had crystal structures or were amorphous was checked. As a result, it was found that the dendrite-shaped crystals and the surface layers of the dendrite-shaped crystals had crystal structures and the intercrystal filling regions were amorphous.

Meanwhile, impurities in the dendrite-shaped crystals were investigated, and the amounts of C, Si, S, Fe, and N were respectively approximately 0.1 to 5 atm %.

The plating layers in the surface-treated steel sheets of Comparative Example x11 to Comparative Example x16 were analyzed using an X-ray diffraction apparatus (XRD: RINT 2500 manufactured by Rigaku Corporation). As a result, for the plating layers in Comparative Example x11 to Comparative Example x16, it was confirmed that the dendrite-shaped crystals had Zn crystal structures. In addition, cases in which the intercrystal filling regions were not formed (Comparative Example x15) or cases in which only unstable amorphous diffraction patterns could be obtained even when the intercrystal filling regions were formed (Comparative Examples x11 to x14 and x16) were confirmed.

In addition, for the plating layers in the examples and the comparative examples, the following items were evaluated using methods described below.

(Adhered Amount in Plating Layer and Content Ratio of Zirconium)

As the adhered amount of the plating layer, the total mass of a Zn element and a Zr element per unit area which were detected using a fluorescent X-ray apparatus (Simultix 14 manufactured by Rigaku Corporation) was used. In addition, the amount of zirconium in the plating layer was computed as a percentage by dividing the amount of the Zr element detected using the fluorescent X-ray apparatus by the adhered amount.

TABLE 18 Plating Corrosion resistance Adhered Zr content Sacrificial Powdering amount ratio Barrier protection property Example g/m² % by mass property property (mm) Z1 5.0 13 3 3 0 Z2 5.0 33 5 1 5 Z3 5.0 22 5 2 2 Z4 5.0 17 5 2 1 Z5 5.0 16 4 2 2 Z6 5.0 16 4 2 0 Z7 5.0 13 3 2 0 Z8 5.0 24 5 2 1 Z9 5.0 17 4 2 1 Z10 5.0 16 4 2 0 Z11 5.0 22 5 2 1 Z12 5.0 29 5 2 2 Z13 5.0 13 2 3 0 Z14 10.0 19 5 3 1 Z15 20.0 22 5 4 2 Z16 7.5 19 5 3 1 Z17 2.5 17 4 1 0 Z18 5.0 17 5 4 1 Z19 5.0 17 5 5 1 Z20 5.0 17 5 5 1

TABLE 19 Plating Corrosion resistance Adhered Zr content Sacrificial Powdering Comparative amount ratio Barrier protection property Example g/m² % by mass property property (mm) x11 5 6 1 2 2 x12 6 4 1 4 1 x13 5 5 2 3 1 x14 5 6 1 2 2 x15 20 0 1 5 0 x16 5 7 1 3 0

As shown in Table 18 and Table 19, it was found that all of Example Z1 to Example Z20 satisfied the scopes of the present invention and had a superior barrier property and superior coating film adhesion compared with the surface-treated steel sheets of Comparative Example x11 to Comparative Example x16.

In addition, in the plating layers in the surface-treated steel sheets of Example Z1 to Example Z20 in which the current densities were 0 to 18 A/dm² in the base-material-forming process, unlike the plating layers in the surface-treated steel sheets of Comparative Examples x12 and x13 in which the current densities were 25 A/dm² in the base-material-forming process, the molar ratio (Zr/Zn) of zirconium to zinc in the intercrystal filling regions reached 1.00 or more and 3.00 or less, and the intercrystal filling regions showed amorphous diffraction patterns when electron beam diffraction was carried out. In addition, it was found that the barrier property and the coating film adhesion were superior.

Example 4

“Test Results of Film-Forming Zirconium-Containing Surface-Treated Steel Sheet”

A coating composition for forming films was prepared by stirring and dispersing the organic resin (R), the phosphoric acid compound (P), the carbon black (C), the organic silicon compound (W), the fluoro metal complex compound (F), the isocyanate compound (I), and the polyethylene wax (Q) shown in Table 8 in water which was a solvent at amounts (% by mass of the solid contents) shown in Table 20 and Table 21 using a coating disperser.

Meanwhile, the same facts as in Example 2 will not be described.

TABLE 20 Phosphoric acid Organic silicon Fluoro metal Organic compound Concentration Carbon compound (W) complex Isocyanate Poly- resin (P) of phosphoric black (W1)/ compound compound ethylene R/ Coating Number Plating (R) P1 P2 acid ions (C) (W1) (W2) (W2) (F) (I) wax (Q) W stability Example u1 Z4 39 6 — — 0 25 25 1.0 — — 5 0.8 OK Example u2 Z4 37 6 4  9.3 0 24 24 1.0 — — 5 0.8 OK Example u3 Z4 35 6 9 13.5 0 23 22 1.0 — — 5 0.8 OK Example u4 Z4 33 6 9 13.5 5 21 21 1.0 — — 5 0.8 OK Example u5 Z4 30 6 9 13.5 10 20 20 1.0 — — 5 0.8 OK Example u6 Z4 28 6 9 13.5 15 19 18 1.1 — — 5 0.8 OK Example u7 Z4 25 6 9 13.5 20 18 17 1.1 — — 5 0.7 OK Example u8 Z4 45 6 9 13.5 10 13 12 1.1 — — 5 1.8 OK Example u9 Z4 50 6 9 13.5 10 10 10 1.0 — — 5 2.5 OK Example u10 Z4 40 6 9 13.5 10 10 10 1.0 — 10 5 2.0 OK Example u11 Z4 40 6 9 13.5 10 8 7 1.1 5 10 5 2.7 OK Example u12 Z4 35 6 9 13.5 10 8 7 1.1 5 15 5 2.3 OK Example u13 Z4 35 6 9 13.5 10 8 7 1.1 5 15 5 2.3 OK Example u14 Z4 35 6 9 13.5 10 8 7 1.1 5 15 5 2.3 OK

TABLE 21 Phosphoric acid Organic silicon Fluoro metal Organic compound Concentration Carbon compound (W) complex Isocyanate Poly- resin (P) of phosphate black (W1)/ compound compound ethylene R/ Coating Number Plating (R) P1 P2 ions (C) (W1) (W2) (W2) (F) (I) wax (Q) W stability Comparative y1 x13 35 6 9 13.5 0 23 22 1.0 — — 5 0.8 OK Example Comparative y2 x13 45 6 9 13.5 10 13 12 1.1 — — 5 1.8 OK Example Comparative y3 x13 40 6 9 13.5 10 10 10 1.0 — 10 5 2.0 OK Example Comparative y4 x13 40 6 9 13.5 10 9 8 1.1 3 10 5 2.4 OK Example Comparative y5 x13 40 6 9 13.5 10 9 8 1.1 — 10 5 2.4 OK Example Reference f1 Z4 No film Example

As shown in Table 20 and Table 21, the coating compositions that were used in Examples u1 to u14 and Comparative Examples yl to y5 were evaluated as “OK” in terms of the coating stability and had excellent stability.

Next, films were formed on the surface of the plating layer in the surface-treated steel sheet of Example Z4 or Comparative Example x13 manufactured in (Example 3) respectively using the coating compositions and method described below.

First, the coating compositions were applied to the surface of the surface-treated steel sheet using a roll coater so as to obtain film thicknesses shown in Table 22 and Table 23. After that, the surface-treated steel sheet to which the coating compositions had been applied was heated and dried so that the sheet reaching temperature reached 150° C. and was spray-cooled using water, thereby obtaining films. Meanwhile, even after the heating to 150° C., hydrated oxides were present in the plating layers.

Next, for the respective surface-treated steel sheets having the film formed on the surface of the plating layer, the external appearance evenness, the corrosion resistance, the electric conductivity, the working adhesion, and the scratch resistance were respectively evaluated. In addition, as Reference Example f1, the external appearance evenness, corrosion resistance, electric conductivity, working adhesion, and scratch resistance of the surface-treated steel sheet of Example Z4 manufactured in (Example 3) were evaluated.

The evaluation results of the respective items are shown in Table 22 and Table 23.

TABLE 22 After forming of film Film External thickness appearance Corrosion Electric Working Scratch Number μm evenness resistance conductivity adhesion resistance Example u1 1 2 4 5 5 2 Example u2 1 2 5 5 5 2 Example u3 1 2 6 5 5 2 Example u4 1 3 5 5 5 2 Example u5 1 4 5 5 5 2 Example u6 1 4 4 5 5 2 Example u7 1 4 4 5 5 2 Example u8 1 4 5 5 5 3 Example u9 1 4 5 5 5 3 Example u10 1 4 5 5 5 3 Example u11 1 4 5 5 5 5 Example u12 1 4 5 5 5 5 Example u13 3 4 6 4 5 5 Example u14 5 4 6 1 5 4

TABLE 23 After forming of film Film External thickness appearance Corrosion Electric Working Scratch Number μm evenness resistance conductivity adhesion resistance Comparative y1 1 1 1 5 1 1 Example Comparative y2 1 1 1 5 1 1 Example Comparative y3 1 1 1 5 1 1 Example Comparative y4 1 1 1 5 1 1 Example Comparative y5 1 1 1 5 1 1 Example Reference f2 — 2 1 6 — — Example

As shown in Table 22 and Table 23, in Examples u1 to u14 in which the film was provided on the surface of the plating layer in the surface-treated steel sheet of Example Z4, the evaluation results were 2 or higher in terms of the evaluation standards in all of the evaluation items, and excellent external appearance evenness, corrosion resistance, electric conductivity, working adhesion, and scratch resistance were showed. In addition, compared with Reference Example f1 in which no film was formed on the surface, Examples u1 to u14 had favorable corrosion resistance and favorable electric conductivity.

Meanwhile, in Comparative Examples y1 to y5 in which the film was provided on the surface of the plating layer in the surface-treated steel sheet of Comparative Example x13, the evaluation results were 1 in terms of the external appearance evenness, the corrosion resistance after 240 hours, the working adhesion, and the scratch resistance, and the performance was poor.

Hitherto, the preferred embodiments of the present invention have been described, but it is needless to say that the present invention is not limited to such examples. It is evident that those skilled in the art are capable of imagining a variety of modification examples or correction examples within a scope described in the claims, and it is needless to say that these examples are understood to belong to the technical scope of the invention.

INDUSTRIAL APPLICABILITY

According to the respective embodiments, it is possible to provide a surface-treated steel sheet having an excellent barrier property and excellent coating film adhesion in which a plating layer including zinc and vanadium is formed on the surface of a steel sheet.

BRIEF DESCRIPTION OF THE REFERENCE SYMBOLS

1, 201 steel sheet

2 plating bath

2 a upper portion supply pipe

2 b lower portion supply pipe

2 c, 2 e outer circumferential branching path

2 d, 2 f intermediate branching path

3 positive electrode

3 a inside of dendrite-shaped crystal

3 b, 55, 58 surface layer of dendrite-shaped crystal

3 c granular crystal

4 a, 5 a, 4 b, 5 b roll

6 vanadium compound

10, 210 surface-treated steel sheet

20, 51, 220 base-material layer

20 a nickel plating layer

21 plating tank

21 a upper portion tank

21 b lower portion tank

30, 230 plating layer

31, 53, 56, 231 dendrite-shaped crystal

32, 52, 54, 57, 232 intercrystal filling region

40, 240 surface layer

61 current-concentrating portion

62 hydrogen

250 amorphous layer

D interval between roll 4 a, 5 a and positive electrode 3 

1-9. (canceled)
 10. A surface-treated steel sheet comprising: a steel sheet; and a plating layer which is formed on one surface or both surfaces of the steel sheet and which includes zinc and one of the group consisting of vanadium and zirconium, wherein the plating layer includes dendrite-shaped crystals including metallic zinc, and intercrystal filling regions which fill spaces between the dendrite-shaped crystals and show amorphous diffraction patterns when electron beam diffraction is carried out, wherein when the plating layer includes the vanadium, the intercrystal filling regions include a hydrated vanadium oxide or a vanadium hydroxide, and wherein when the plating layer includes zirconium, the intercrystal filling regions include a hydrated zirconium oxide or a zirconium hydroxide.
 11. The surface-treated steel sheet according to claim 10, further comprising: a base-material layer made of crystals including nickel between the steel sheet and the plating layer.
 12. The surface-treated steel sheet according to claim 10, wherein, when the plating layer includes the vanadium, V/Zn which is a molar ratio of the vanadium to the zinc in the intercrystal filling regions is 0.10 or more and 2.00 or less, and wherein when the plating layer includes the zirconium, Zr/Zn which is a molar ratio of the zirconium to the zinc in the intercrystal filling regions is 1.00 or more and 3.00 or less.
 13. The surface-treated steel sheet according to claim 10, wherein the plating layer includes the vanadium, and surface layers of the dendrite-shaped crystals include a zinc oxide or a zinc hydroxide.
 14. The surface-treated steel sheet according to claim 10, further comprising: a base-material plating layer having Zn/V, which is a molar ratio of zinc to vanadium, of 8.00 or more between the steel sheet and the plating layer.
 15. The surface-treated steel sheet according to claim 11, further comprising: a base-material plating layer having Zn/V, which is a molar ratio of zinc to vanadium, of 8.00 or more between the base-material layer and the plating layer.
 16. The surface-treated steel sheet according to claim 10, further comprising: an organic resin film having a polyurethane resin and 1% to 20% by mass of carbon black on a surface of the plating layer.
 17. A method for manufacturing the surface-treated steel sheet according to claim 10, the method comprising: a base-material-forming process of forming protrusions and recesses by precipitating a hydrated vanadium oxide or a vanadium hydroxide on the steel sheet in a case in which a plating layer comprises vanadium, or by precipitating a hydrated zirconium oxide or a zirconium hydroxide on the steel sheet in a case in which a plating layer comprises zirconium, by carrying out an electroplating at an average flow rate of 40 to 200 m/min and a current density of 0 to 18 A/dm² using a plating bath containing 0.10 to 4.00 mol/l of Zn²⁺ ions and one of the group consisting of 0.01 to 2.00 mol/l of V ions and 0.10 to 4.00 mol/l of Zr ions; and an upper layer plating process of carrying out an electroplating on the steel sheet on which the protrusions and the recesses are formed at an average flow rate of 40 to 200 m/min and a current density of 21 to 200 A/dm² using the plating bath.
 18. The method for manufacturing a surface-treated steel sheet according to claim 17, further comprising: before the base-material-forming process, a process of precipitating crystals including nickel on the steel sheet by carrying out an electroplating using a plating bath containing Ni²⁺ ions. 